!=> 


t  ' 


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TREATISE 

ON 

PNEUMATICS: 

BEING 

THE    PHYSICS    OF    GASES, 

INCLUDING    VAPORS. 

CONTAINING 

A    FULL   DESCRIPTION   OF   THE   DIFFERENT  AIR  PTTMPS,  AND  THE  EXPERIMENTS  WHICH   MAY  BE 
PERFORMED  WITH  THEM ;  ALSO  THE  DIFFERENT  BAROMETERS,   PRESSURE  GAUGES, 

HYGROMETERS,  AND  OTHER  METEOROLOGICAL  INSTRUMENTS, 

EXPLAINING   THE   PRINCIPLES   ON  WHICH   THEY  ACT,  AND 
THE   MODES   OF   USING   THEM. 

Illustrated  fij  Nunurous  jFiite  SEooir  3%nzxzbin%8. 

BY 

MARTIN"  H.  BOYE,  M.D.A.M. 

PROFESSOR    OF   NATURAL   PHILOSOPHY    AND    CHEMISTRY  IN    THE    CENTRAL    HIGH   SCHOOL    OF   PHILADELPHIA, 

FORMERLY  ASSISTANT  GEOLOGIST  AND  CHEMIST  TO   THE    GEOLOGICAL   SURVEY  OF  TH  3  STATE  OF 

PENNSYLVANIA,   MEMBER   OF  THE   AMERICAN  PHILOSOPHICAL  SOCIETY,   ETC.  ETC. 


PHILADELPHIA: 
E.  C.  &  J.  BLDDLE,  No.  8  MINOR  STREET, 

(Between  Marlcet  and  Chestnut,  and  Fifth  and  Sixth  Sts.) 

1855. 


i.    (       ^    Al     . 


Entered  according  to  the  Act  of  Congress,  in  the  year  1855,  by 
M.  H.  BOYE, 

in  the  Clerk's  Office  of  the  District  Court  of  the  United  States  for  the  Eastern 
District  of  Pennsylvania. 


STEREOTYPED  BY  L.  JOHNSON  &  CO. 
PHILADELPHIA. 


Printed  by  T.  K.   &  P.  G    Collins. 


Q'OICI 


PREFACE. 


THE  frequent  inquiries  made  in  regard  to  the  principles,  differ- 
ent constructions,  and  modes  of  using  the  different  meteorological 
instruments,  which  come  within  the  subject  treated  of  in  this 
little  volume,  and  the  general  and  increasing  interest  felt  in  these 
matters,  induce  the  author  to  believe  that  the  present  work  will 
supply  a  want  which  has  been  much  felt.  While  he  has  adhered 
to  a  strict  systematic  arrangement,  and,  on  the  part  of  science, 
sacrificed  nothing  to  popularity,  he  hopes  that  he  has  made  the 
explanations  so  clear  and  full  as  to  be  intelligible  to  all.  Nor 
has  he  spared  any  trouble  or  expense  in  illustrating  the  subject  by 
numerous  appropriate  wood-cuts  made  expressly  for  this  work,  and 
many  of  them  entirely  original.  For  the  use  of  the  different  instru- 
ments a  series  of  Tables  has  been  added,  including  those  of  the 
Tensions  of  Vapor  of  Water,  used  with  the  Boiling-Point  Barome- 
ter and  the  different  Hygrometers,  which  Tables  have  been  calcu- 
lated for  this  work  from  those  of  Regnault,  and  are  here  given, 
for  the  first  time,  complete  in  English  measures  and  Fahrenheit 
degrees. 

PHILADELPHIA,  May  IQlh,  1855. 


CONTENTS. 


ON  INANIMATE  MATTER. 


GENERAL   INTRODUCTION. 
Paragr.  Page 

1.  Matter.  Sciences.  Physical  Sciences.     9 

2.  Forces.     Laws.     Object  of  Physical 

Sciences 9 

3.  Life.     Physics   of  Animate   Matter 

or  Physiology 9 

4.  Physics  of  Inanimate  Matter,  how  di- 

vided   9 

5.  Descriptive  Sciences 10 

6.  Applied  or  Practical  Sciences 10 

7.  Mixed  Sciences 10 

DIVISION  I. 

PHYSICS  PROPER,  OR  NATURAL  PHILOSOPHY. 
INTRODUCTION. 

8.  Physics  proper  defined 11 

9.  Plan  of  distribution  of  Matter  through 

Space 11 

10.  Ultimate    construction    of    matter. 

Atoms 11 

11.  Cohesion 12 

12.  Different  Forms  or  States  of  Matter. 

Solid,  liquid  and  gaseous  states 12 

14.  Ether,  or  Imponderable  Matter 13 

15.  Adhesion 13 

16.  Gravity 13 

17.  Impenetrability 14 

18.  Impact  or  Impulse 14 

19.  Inertia 14 

20.  Limit.     Form.     Numbers 14 

21.  Motion  and  Best 15 

22.  Their  relation  to  matter 15 

23.  Physics  proper,  how  divided 15 

General  Table  of  Divisions  and  Sub- 
divisions of  Physical  Sciences 16 

PART  I, 

PHYSICS    OF   PONDERABLE    MATTER. 

SECTION  I. 

PNEUMATICS,  OR   PHYSICS    OP    GASES. 
Properties  depending  on  Cohesion. 

24.  Expansibility 17 


Paragr.  Page 

25.  The  Atmosphere 17 

26.  Different  gases  of  the  Atmosphere...  18 

27.  Extent  of  Atmosphere 18 

28.  Exhausting  Air-pumps.     Single-bar- 

relled stopcock -pump 18 

29.  Mode  of  Action.     Injurious  Space...  20 

30.  Single-barrelled  Valve-pump  or  Sy- 

ringe   21 

31.  Wide-mouthed  Receivers  and  Plate...  21 

32.  Double-barrelled    Exhausting    Air- 

pump 21 

33.  Single-barrelled,  double-acting 22 

34.  Improved    single-barrelled,    single- 

acting 24 

35.  Mode  of  calculating  rarefaction 26 

37.  Suction  by  the  mouth 27 

38.  Other  means  of  exhaustion.     Filling 

of  Thermometer-bulbs 27 

39.  Compressibility    and    Elasticity    of 

Gases 27 

40.  Forcing  or  Condensing  Air-pumps....  28 

41.  Single-barrelled 28 

42.  Receivers  and  Plate 29 

43.  Single-barrelled,  double-acting 30 

44.  Mariotte's  Law 30 

45.  Permanent   and   Liquefiable    gases. 

Vapors 3,0 

48.  Diving  Bell -31 

49.  Air-gun 31 

50.  Other  means  of  compressing  gases. 

Steam-boiler.     Fire-arms 32 

Properties  depending  on  Adhesion. 

52.  Diffusibility  of  Gases 32 

53.  Diffusion  through  Porous  Bodies 33 

54.  Condensation   of    Gases   on    Solids. 

Hygroscopic  Water.    Platinum  Ig- 
niter   33 

55.  Solution  of  Gases  in  Liquids 34 

56.  Diffusion  of  Gases  in  Solution.     Re- 

spiration. Confining  Gases  by  Wa- 
ter and  by  Mercury 35 

Properties  depending  on  Gravity. 

57.  Weight  of  Gases 35 

58.  Specific  Gravity  of  Gases 36 


CONTENTS. 


Paragr.  Page 

59.  Pressure  caused  by  Weight  of  Atmo- 

sphere   37 

60.  Torricellian  Tube 38 

61.  Pressure  of  Atmosphere,  how  esti- 

mated.    Used  as  Unit 38 

62.  Torricellian  Vacuum 39 

The  Barometer. 

63.  Cup  and  Syphon  Barometers 40 

64.  Source  of  Inaccuracy,  how  remedied.  41 
66.  Water  Barometer 42 

68.  Diagonal   or  Inclined  Plane  Baro- 

meter   43 

69.  Wheel  Barometer 43 

70.  Huygen's  Double-Barometer 44 

71.  Means   of  increasing  Accuracy  in 

measuring 44 

72.  Vernier,  its  Nature  and  Construction  44 

75.  Effects  of  Capillarity  on  Barometer..  47 

Table  of  Correction  for  same 47 

76.  Friction  and  Adhesion  of  Mercury...  48 

77.  Expansion  by  Heat  of  Mercury,  and 

of  Scale 48 

78.  Marine  Barometer 50 

79.  Gay-Lussac's  Portable  Syphon  Baro- 

meter   51 

80.  Preventing  Air  getting  into  the  Va- 

cuum   53 

81.  Accurate  Levelling  Barometer 53 

82.  Standard  Barometers 55 

83.  Self-registering 55 

84.  Objections  to  Mercurial  Barometer..  55 

85.  Substitutes  for  Mercurial  Barometer.  56 

86.  Sympiesometer 56 

87.  Boiling-Point  Barometer 57 

88.  Aneroid  Barometer 58 

89.  Metallic  Barometer  (Bourdon's) 61 

90.  Nature  of  Barometer 62 

91.  Manometer 62 

Uses  of  the  Barometer. 

92.  As  Weather-glass 63 

93.  Range  and  Variations  of  Barometer.  65 

94.  For  measuring  Heights.  Principle  of.  65 

Mode  of  Calculating 67 

96.  Example 69 

98.  Rapid   Decrease   in    Pressure  and 

Density  of  the  Atmosphere 70 

99.  Mode   of  taking    Observations   for 

Levelling 70 

100.  Estimating    the     true    Volume    of 

Gases,  and  from  it,  their  Weight..  71 

Mariotte's  Law. 
101..  Experiments    to  prove    Mariotte's 

Law 73 

103.  Mariotte's  Tube 75 

104.  Exceptions  to  Mariotte's  Law 75 

Pressure-gauges. 

105.  Mercurial  Exhaustion-gauge  for  Air- 

pumps 75 


Paragr.  Page 

106.  Mercurial  Pressure-gauges 76 

107.  Condensed-Air  Pressure-gauges 77 

108.  For  very  high  Pressures 78 

109.  Of  very  small  Dimensions 79 

110.  Steam-gauges  (Manometers).  Safe- 

ty-valves   79 

Ezperiments  to  illustrate  Pressure  of 
Atmosphere. 

111.  Fountain  in  Vacuo 80 

112.  Mercurial  Rain 80 

113.  Bursting  of  Bladder '80 

114.  Upward  Pressure  of  Atmosphere....  80 

115.  Magdeburg  Hemispheres 81 

116.  Pressure  on  Human  Body 81 

Experiments  to  illustrate  Expansibility,  Elas- 
ticity, and  Compressibility  of  Atmospheric 
Air. 

117.  Difference    between    Expansibility 

and  Elasticity.     Tension 82 

118.  Inflation  of  Bladder  by  Expansi- 

bility   83 

119.  Mechanism  of  Respiration 83 

120.  Expulsion  of  Air   from  Water   by 

Exhaustion 84 

121.  From  the  Pores  of  Charcoal.     Their 

filling  with  water Si 

122.  Hero's  Ball 84 

123.  Condensed- Air  Chamber  of  Hydrau- 

lic Engines 85 

Impact  and  Inertia  of  Gases. 

124.  Resistance  of  Air.     Windmill  Ex- 

periment   85 

125.  In  a  Vacuum  all  Bodies  fall  equally 

fast.     Feather  and  Guinea  Ex- 
periment  ,  86 

126.  Resistance   of   Air  to   Projectiles. 

Flight  of  Birds 87 

127.  Winds.     Table  of  their   Velocities 

and  Force 87 

128.  Anemometer 88 

129.  Efflux    of    Gases    into   a   Vacuum 

through  Capillary  Orifices  (Effu- 
sion)   88 

130.  Through  Capillary  Tubes  (Transpi- 

ration)   89 

131.  Efflux  of  Gases  into  the  Atmosphere.  89 

132.  Revolving  Gas  Jet 89 

133.  Pneumatic  Paradox 90 

VAPORS. 

134.  Circumstances  under  which  formed.  91 

135.  Nature  of  Vapors 91 


Formation  of  Vapors  in  a  Va 

136.  Has  a  Limit.  Maximum  Quantity 
and  Tension;  depends  on  Tem- 
perature   92 


CONTENTS. 


vii 


Paragr.  Page. 

137.  Quantity  of  Vapor  estimated  from  its 

Tension 93 

138.  Table  of  Maximum  Quantities  and 

Tensions 93 

139.  Tension  or  Elasticity  of  Steam  at 

high  Temperatures 93 

140.  Expansion  of  Vapors  by  Heat 94 

141.  Vapors  not  filling  space  to  Satura- 

tion may  be  subjected  to  Pressure 
and  Cold 95 

142.  Illustration  as  regards  Pressure 95 

143.  As  regards  Cold.  Dew-point.  Proof 

of  Saturation 96 

144.  Boiling  in  a  Vacuum,  how  produced.  96 

145.  Culinary  Paradox 97 

146.  Papin's  Digester 97 

147.  Theoretical  Stop  to  Evaporation...  97 

148.  Different  Volatility  of  Substances..  98 

149.  Modes  of  increasing  Evaporation  in 

a  Vacuum 98 

150.  Applications  in  Chemistry 98 

Formation  of  Vapors  in  a  Gas. 

151.  Has  a  Limit.   Maximum  Quantities 

and  Tensions  the  same  as  in  a 
Vacuum 99 

152.  Difference  between  the  Formation  of 

Vapors  in  a  Gas  and  in  a  Vacuum  100 

153.  Boiling  in  Open  Air.     Simmering.  100 

154.  Boiling-Point    of    different    Sub- 

stances    101 

155.  Boiling-Point  altered  by  change  in 

Pressure 101 

156.  Limit  to  Boiling  in  a  Vacuum 101 

158.  Modes  of  increasing  Evaporation  in 

a  Gas 101 

159.  Applications  in  Chemistry 102 

Vapor  of  Water  in  the  Atmosphere. 

160.  Different   States   in  which  Water 

exists  in  the  Atmosphere.  Dew. 
Fogs.  Clouds ;  different  Varie- 
ties of.  Rain;  Rain-gauge  or 
Ombrometer.  Hail.  Snow 102 

161.  How  Vapors  affect  the  Atmosphere.  103 

162.  Moisture  or  Humidity  of  the  Atmo- 

sphere. Relative  Moisture  or 
Humidity 103 

163.  Amount  of  Vapor,  how  estimated 

by  Chemical  Method...  „ 104 

Hygrometers. 

164.  Their  use.     Mode  of  finding  the 

Relative  Humidity  from  the 
Dew-point  and  the  Temperature 
of  the  Atmosphere 105 

165.  To  find  Tension  of  Vapor  and  Dew- 

point  from  Relative  Humidity 
and  Temperature  of  Atmosphere  106 

166.  To  find  per  centage  of  Vapor  by 

Volume ..  106 


Paragr.  Page. 

167.  Per  centage  of  Vapor  by  Weight...  106 

168.  Absolute  Weight  of  Vapor  in  a  cer- 

tain Volume 107 

Hygrometers  giving  the  Dew-point. 

169.  Darnell's  Hygrometer 108 

170.  Bache's  Hygrometer 109 

171.  Regnault's  Hygrometer 109 

172.  August's  Psychrometer,  or  the  Wet- 

Bulb  Hygrometer 110 

173.  Formula  for  Tension  of  Vapor,  and 

Dew-point,  from  Wet-Bulb  Hy- 
grometer   Ill 

174.  Example 112 

175.  Precautions    in    using    Wet-Bulb 

Hygrometer 113 

Hygrometers  acting  by  Absorption. 

177.  Their  mode  of  Action 113 

178.  Saussure's  Hair  Hygrometer 114 

179.  Table  of  Relative  Humidities  cor- 

responding to  its  Degrees 115 

180.  Objections  to  the  Hair  Hygrometer  115 

181.  Hygroscopes    made  from  Whale- 

bone, Wood,  Twisted  Strings, 
Beard  of  Sensitive  Oats,  Blad- 
der, &c 115 

Tables. 

TABLE  I.  Correction  for  Temp,  for  Barome- 
ters mounted  in  Wood. 

TABLE  II.  Correction  for  Temp,  for  Baro- 
meters with  Brass  Scale,  extending  the 
whole  length. 

TABLE  III.  For  finding  differences  in  Height 
between  two  Places  from  Barometric  Ob- 
servations. 

TABLE  IV.  Correction  of  same  for  Latitude. 

TABLE  V.  Correction  of  same  for  Altitude. 

TABLE  VI.  Conversion  of  French  into  Eng- 
lish, and  of  English  into  French  mea- 
sures, &c. 

TABLE  VII.  Maximum  Tension,  or  Elastic 
Force  of  Vapor  of  Water  for  every  0.2  de- 
gree from  214°  to  185.°  For  Boiling-Point 
Barometer. 

TABLE  VIII.  Maximum  Tension  of  Vapor 
of  Water  for  every  degree  from  185°  to 
104°. 

TABLE  IX.  Maximum  Tension,  or  Elastic 
Force  of  Vapor  of  Water,  for  every  0.2 
degree  from  204°  to  0°,  and  for  every  de- 
gree from  0°  to  —  31°.  For  (Dew-Point) 
Hygrometers. 


ON 


INANIMATE  MATTER 


GENERAL  INTRODUCTION. 

1.  BY  Matter  we  understand  all  that  acts  on  our  senses.     Matter,  there- 
fore,  constitutes  the  whole   external  or  Material  World,  the   Universe. 
Our  knowledge  of  matter  systematically  arranged  constitutes  the  Sciences 
of  Matter,  or  the  different  Material  or  Natural  or  Physical  Sciences,  or 
Physics  in  its  widest  sense ;  in  contradistinction  to  the  sciences  of  the 
mind,  or  the  Mental  and   Moral  Sciences,  treating  of  the   internal  or 
immaterial  world. 

2.  The  different  phenomena  and  properties  of  matter  we  account  for 
by  ascribing  them  to  certain  causes  inherent  in  matter  itself,  which  we 
call  Forces.     These  forces  are  always  found  to  act  according  to  certain 
rules  or  laws.     The  main  object  of  the  physical  sciences  must,  therefore, 
be  to  discover  these  forces,  and  expose  the  laws  according  to  which  they 
act. 

3.  But  besides  the  general  forces,  which  all  matter  obeys  at  all  times, 
matter  is  also  capable  of  being  brought  under  a  peculiar  influence,  which 
we  call  Life.     While  under  such  influence  it  is  called  Animate  matter,  in 
contradistinction  to  which,  when  not  under  this  influence,  it  is  called 
Inanimate  matter.     We  therefore  get  two  main  branches  of  the  physical 
sciences:  Physics   of  Animate  matter,  or  Physiology  (Special   Physics), 
which  treats  of  Life  and  the  manner  in  which  matter  is  influenced  by  it, 
or  of  Animate  matter;  and  Physics  of  Inanimate  matter  (General  Physics), 
which  treats  of  Inanimate  matter. 

4.  Our  knowledge  of  inanimate  matter  must  refer  either  to  its  place,  or 
to  its  nature;   we  therefore  get  two  divisions  of  Physics  of  Inanimate 


10  BOYE'S   INANIMATE   MATTER. 

matter,  Physics  proper,  or  Natural  Philosophy,  which  treats  of  the  place 
of  inanimate  matter,  and  Chemistry,  which  treats  of  its  nature. 

5.  The  physical  sciences  have  each  their  descriptive  part,  describing 
the  different  objects  or  bodies  formed  in  nature  by  the  forces  or  influences 
of  which  they  treat.     These  descriptive  parts  are  often  considered  as  sepa- 
rate sciences,  and  called  the  sciences  of  Objects,  in  contradistinction  to 
which  the  others,  of  which  they  are  only  descriptive  parts,  are  called  the 
sciences  of  Phenomena.     Thus  the  descriptive  part  of  Physiology,  or  a 
description  of  all  the  different  forms  of  matter  assumed  under  the  influ- 
ence of  life,  or  animate   objects,  constitutes   Natural  History,  of  which 
again  Anatomy  is  a  subordinate  branch.     Uranography  is  a  descriptive 
part  of  Physics,  Mineralogy   of  Chemistry,  and   Geology,    Meteorology 
and  Physical  Geography,  are  descriptive  parts  of  Physics  and  Chemistry. 

6.  The  above  main  branches  and  divisions  of  the  natural  sciences,  when 
applied  to  particular  purposes,  as  the  performance  of  certain  mechanical 
operations,  or  the  production  of  certain  chemical  compounds,  required  for 
our  necessities  or  comforts  or  other  relations  of  social  life,  constitute  the 
different  applied,  or  practical,  or  industrial  sciences.     These  are  used  in 
the  different  trades,  manufactures  and  arts,  and  a  systematic  arrangement 
of  the  greater  number  of  them  is  often  called  Technology.     Agriculture, 
Surgery,  Medicine,  &c.,  are  instances  of  applied  branches  of  Physiology, 
in  connection  with  Physics  and  Chemistry. 

7.  The   different   economical,   political   and   philological    sciences    are 
combinations  of  mental  and  physical  sciences,  pure  or  applied. 


10 


DIVISION  I. 
PHYSICS  PROPER,  OR  NATURAL  PHILO  SOPHY. 


INTRODUCTION. 

IYSICS  proper,  it  lias  been  said  in  a  general  way,  treats  of  the  place 
of  matter.  But  as  we  ^ount  for  all  phenomena  connected  with  matter 
by  ascribing  them  to  certain  causes  inherent  in  matter,  which  we  call 
forces,  which  forces  always  act  according  to  certain  rules  or  laws.  Physics 
proper  must  treat  of  the  forces,  by  which  matter  holds  its  place  in  space, 
and  expose  the  laws  according  to  which  they  act. 

9.  That  which  first  strikes  us  in  regard  to  the  place  of  matter  on  a 
large  scale,  is,  that  we  do  not  find  it  to  be  equally  distributed  through 
space,  but  collected  in  large  masses,  constituting  the  heavenly  bodies  and 
the  earth,  and  the  space  between  them  to  be  comparatively  void. 

10.  "We  have  reason  to  believe,  that  internally  matter  is  constructed  on 
a  similar  plan,  so  that  any  portion  of  it  does  not  consist  of  matter  uni- 
formly diffused  through  the  space  which  it  occupies,  but  that  the  matter 
of  which  it  consists  is  collected  in  small  particles  called  atoms,  with  a 
small  space  between  them,  which  is  comparatively  void.     These  ultimate 
particles   or   molecules  are  called   atoms   (from   a  privative   and   re//vd> 
(temrio)  I  cut),  meaning  what  can  not  be  cut  or  divided,  because  they 
are  considered  to  be  indivisible  and  indestructible.     Though  practically 
they  may  be  considered  infinitely  small,   still  in   reality   they  have   a 
certain  definite  size  and  form.     Their  form  is  generally  considered  to  be 
that  of  small  solid  spheres  or  spheroids,  inside  perfectly  uniform;  single 
spheres  for  simple  bodies  and  clusters  of  such  spheres   for   compound 
bodies.* 

*  The  main  arguments  in  favor  of  the  existence  of  atoms  with  spaces  between  them 
are;  the  general  nature  of  chemical  combination  with  the  laws  of  definite  and  multiple 
proportions,  isomerism  and  allotropism,  for  which  see  under  Chemistry;  cleavage  and 
crystallization,  see  under  Stereotics;  expansibility  of  gases,  see  Pneumatics;  the 

11 


12  BOYE'S   INANIMATE   MATTER. 

11.  All  bodies,  by  which  we  understand  limited  portions  of  matter,  must 
therefore  consist  of  aggregations  of  such  atoms.  These  atoms  being  not 
in  contact  are  kept  at  certain  extremely  small  but  definite  distances  from 
each  other  by  two  forces,  an  attractive  force,  which  tends  to  approach 
them  to  each  other,  and  a  repulsive  force,  which  tends  to  separate  them. 
The  resulting  effect  of  these  two  forces  is  called  COHESION,  and  constitutes 
the  force  with  which  each  atom  is  held  in  the  same  relative  position  to 
the  other  atoms  of  the  same  kind  of  matter.  Compressibility  and  Elas- 
ticity are  properties  of  matter  depending  on  this  same  force. 

.  According  to  the  greater  or  less  strength  of  the  above  attractive  and 
repulsive  forces  between  the  atoms,  constituting  the  degree  of  cohesion, 
matter  presents  itself  in  one  or  the  other  of  the  following  three  states  or 
forms. 

1st.  The  solid  state.  Whenever  the  attractive  and  repulsive  forces 
between  the  atoms  are  great,  the  atoms  are  kept  firmly  in  their  relative 
position,  so  that  they  offer  considerable  resistance  to  any  force  that  tends 
to  move  them  among  themselves,  or  to  separateihem  from  each  other. 
In  this  case,  therefore,  the  cohesion  is  said  to  be  great,  and  the  matter 
presents  itself  in  the  solid  state. 

2d.  The  liquid  state.  In  this  state  matter  presents  itself  when  the 
attractive  and  repulsive  forces  between  the  atoms  are  but  small.  The 
atoms  are  then  held  in  their  relative  position  with  but  a  slight  force,  so 
that  they  can  easily  be  moved  among  themselves,  or  separated  from  each 
other.  In  liquids,  therefore,  the  cohesion  is  small. 

3d.  The  gaseous  state.  This  state  matter  assumes  when  the  atomic 
repulsive  force  is  greater  than  the  attractive.  The  atoms  then  have  a 
tendency  to  separate  from  each  other  and  spread  themselves  through 
space,  unless  prevented  by  some  other  cause.  This  property  in  gases  is 
called  Expansibility,  and  distinguishes  them  from  liquids.  Cohesion  in 
this  case  is  said  to  be  negative.  They  also  offer  little  or  no  resistance  to 
the  motion  of  their  particles  among  themselves,  in  which  point  they  re- 
semble liquids.  For  this  reason  liquids  and  gases  are  comprised  together 
under  the  common  name  of  fluids  in  contradistinction  to  solids. 

13.  One  and  the  same  kind  of  matter  may  often,  under  different  circum- 
stances, exist  in  either  of  the  above  three  states.  Thus  water  when  ex- 
posed to  cold  becomes  solid  or  ice,  and  by  heat  may  be  converted  into  gas 
or  steam.  But  matter  can  only  exist  in  one  state  at  the  same  time,  and 
under  the  same  circumstances  it  nearly  always  assumes  the  same  state. 

expansion  of  all  matter  by  heat,  see  Thermics ;  and  the  undulatory  nature  of  light,  and 
its  passage  through  all  forms  of  ponderable  matter,  see  Photics.  For  the  particulars 
regarding  the  form,  size  and  weight  of  atoms,  see  under  Stereotics. 

12 


PHYSICS   PROPER,    OR  NATURAL   PHILOSOPHY.  13 

14.  The  ethereal  state.  The  existemce  of  a  fourth  state  of  matter  is  ren- 
dered highly  probable,  filling  the  spaces  between  the  atoms  of  the  above 
three  states  (the  interatomic  spaces),  and  the  spaces  between  the  planets 
and  between  the  stars  (the  interplanetary  and  interstellar  spaces).  This 
state  is  called  the  Ethereal,  and  the  matter  itself  Ether.*  That  Ether 
must  differ  materially  from  other  states  of  matter,  follows  from  the  fact, 
that  it  fills  the  spaces  between  their  atoms.  Either  therefore  it  can  ,not 
be  composed  of  similar  ultimate  atoms,  or  these  atoms  must  at  least  be  of 
a  much  smaller  size.  As  it  has  been  found  to  offer  a  sensible  resistance 
to  the  comets  in  their  motion,  it  must,  as  it  will  afterwards  be  understood, 
possess  inertia  and  in  this  point  resemble  the  other  kinds  of  matter.  If, 
however,  it  is  affected  by  gravity  so  as  to  possess  weight,  (see  further  on), 
this  is  so  inconsiderable,  that  it  cannot  be  ascertained  by  the  same  means 
by  which  it  is  proved  for  other  matter,  hence  it  is  generally  called  Im- 
ponderable matter,  in  contradistinction  to  which  the  other  states  of  matter 
are  called  Ponderable  matter.  It  has  not  been  ascertained  whether  other 
states  of  matter  may  also  exist  in  the  Ethereal  state,  or  the  Ether  itself 
be  condensed  or  converted  into  the  others.  On  the  whole,  though  its 
existence  is  well  established,  our  knowledge  of  its  nature  is  yet  but  very 
imperfect. 

15.  The  same  attractive  and  repulsive  forces,  which  exist  between  the 
atoms  of  matter  of  the  same  kind,  we  also  find  between  the  atoms  of 
different  kinds  of  matter,  by  which  these  are  held  in  their  relative  posi- 
tion at  a  small  distance  from  each  other.     The  resulting  effect  is  in  this 
case  called  ADHESION,  because  if  after  having  brought  the  atoms  of  two 
different  bodies  together,  we  again  attempt  to  separate  them,  particles  of 
the  one  often  remain  by  this  force  attached,  or  adhere  to  the  other.     Thus 
if  we  dip  a  glass  rod  into  water  and  then  again  withdraw  it,  some  of  the 
water  will  adhere  to  the  glass  in  preference  to  cohering  to  the  other  par- 
ticles of  itself.     Capillary  attraction  and  solution  are  caused  by  the  same 
force. 

16.  The  attractions  and  repulsions,  of  which  we  have  spoken  (Cohesion 
-   and  Adhesion),  do  not  extend  perceptibly  beyond  a  very  small  distance, 

probably  not  beyond  the  distance  of  proximate  or  neighboring  atoms. 
We  observe,  however,  another  attractive  force  to  exist  between  atoms  of 
the  same  or  different  kinds  of  matter,  and  acting  also  at  distances  greater 
than  the  distances  of  proximate  atoms,  only  in  a  certain  diminishing  ratio. 

*  The  main  argument  for  the  existence  of  Ether  in  all  these  spaces  and  others,  not 
filled  with  ponderable  matter,  we  have  in  the  passage  through  them  of  light,  which  can  be 
proved  to  be  formed  by  undulations,  which  therefore  require  the  existence  of  an  undula- 
ting medium. 


14  BOYE'S   INANIMATE  MATTER. 

As  it  thus  acts  on  all  the  atoms,  of  which  a  body  consists,  and  at  great 
distances,  it  becomes  also  an  attraction  between  masses  of  atoms,  or  bodies 
towards  each  other.  This  attraction  is  called  GRAVITY,  and  must  there- 
fore be  greater  according  to  the  number  of  atoms  in  the  different  bodies. 
We  thus  find  that  a  very  strong  attraction  exists  by  gravity  between  the 
heavenly  bodies  and  the  earth,  and  between  the  earth  and  all  terrestrial 
bodies  on  or  near  its  surface;  but  it  is  exceedingly  small  between  the 
terrestrial  bodies  themselves,  though  it  can  be  proved  also  to  exist  be- 
tween them  according  to  their  size.  Gravity  has  by  some  been  con- 
sidered as  the  result  of  the  attractions  of  cohesion  and  adhesion,  but 
this  is  Hot  probable;  at  all  events  we  are  not  acquainted  with  a  corres- 
ponding repulsive  force  acting  at  a  distance  like  Gravity. 

17.  It  has  been  stated  that  the  ultimate  atoms  are  considered  solid. 
They  therefore  allow  no  other  atoms  of  the  same,  or  any  other  kind  of 
matter  to  enter  or  occupy  the  same  space  at  the  same  time.     This  property 
of  matter  is  called  IMPENETRABILITY.     The  space  between  the  atoms  may 
be  diminished  (Compressibility),  but  the  atoms  of  the   same  body  can 
never  be  forced  into  each  other  even  by  the  greatest  pressure,  nor  will 
they  allow  the  atoms  of  any  other  body  to  be  forced  into  their  place. 
One  kind  of  matter  may,  however,  allow  the  atoms  of  another  kind  to 
penetrate  with  considerable  facility  into   the   spaces   between  its  atoms, 
while  it  will  resist  with  great  force  the   further  approach    of  its   own 
atoms.     This  property  is  called  Diffusibility,  and  depends  on  the  attrac- 
tion, which  has  been  spoken  of  before  as  Adhesion,  and  is  particularly  ob- 
served between  the  atoms  of  solids  and  liquids,  and  also  between  the  atoms 
of  different  kinds  of  gases. 

18.  When  matter  has  been  influenced  by  a  force  to  move,  and  in  its  way 
meets  other  matter,  so  that  it  can  not  continue  its  motion  without  putting 
this  matter  also  in  motion,  we  find  this  latter  to  take  place,  and  a  portion 
of  its  own  motion  to  be  transferred  to  it.     We  thus  find,  that  motion  is 
transferable  by  IMPACT  or  IMPULSE  from  one  portion  of  matter  to  another. 

19.  Matter  has  also  an  inherent  force  to  preserve  its  state  of  rest  or 
motion.     This  force  or  property  of  matter  is  called  INERTIA,  and  is  gene- 
rally expressed  thus,   that  matter  when  at  rest  cannot  by  itself  begin 
motion,  nor  when  in  motion  can  it  alter  this  so  as  to  pass  to  rest,  or  to  a 
slower  or  faster  motion,  or  in  a  different  direction,  unless  influenced  by 
some  other  cause. 

/  f\    20.  With  the  idea  of  matter  and  its  existence  is  necessarily  given  the 

\   idea  of  space  to  exist  in.     Where  one  kind  of  matter  ceases  and  another 

begins,  there  must  be  a  limit,  and  all  limited  portions  of  matter,  or  bodies, 

must  therefore  have  a  Form.     But  the  abstraction  of  space  and 

14 


"R. 


PHYSICS   PROPER,   OR  NATURAL  PHILOSOPHY.  15 

matter,  and  its  separate  consideration  can  only  be  made  in  the  mind,  and 
constitutes,  therefore,  a  purely  mental  science,  Geometry,  which  does  not 
belong  to  the  natural  sciences,  while  the  application  of  its  results  to  the 
forms  of  matter,  as  they  actually  occur  in  nature,  is  of  the  utmost  import- 
ance to  them  (Crystallography,  &c.).  The  same  is  the  case  with  the 
abstraction  of  the  idea  of  repetition  of  separate  but  like  portions  of  matter 
or  Numbers,  and  their  separate  study,  which  constitutes  Algebra,  and  in 
its  application  is  of  equal  importance  to  the  natural  sciences. 

1.  Matter,  while  it  by  its  own  inherent  forces  influences  other  matter 
and  itself  to  motion,  is  equally  susceptible  to  the  forces  of  all  other  matter, 
and  will  move  under  their  influence.  The  influence  of  a  single  force  is  to 
move  it  in  a  straight  line.  But  as  it  is  always  acted  on  at  the  same  time 
by  a  number  of  forces,  and  has  to  move  according  to  all  of  them,  its 
motion  is  always  more  or  less  complex.  If  at  the  same  time  matter  be 
influenced  by  different  forces  to  move  equally  in  opposite  directions,  it  will 
retain  the  same  place  or  be  at  rest.  Though  experience  teaches  us  that 
all  matter  is  in  constant  motion,  no  particle  retaining  the  same  place  for 
any  length  of  time,  so  that  there  is  no  absolute  rest,  still  a  body  may  be 
influenced  so  as  not  to  alter  its  position  in  regard  to  surrounding  objects, 
and  we  then  generally  say,  that  it  is  at  rest,  though  it  is  only  relative  or 
apparent  rest. 

22.  As  the  amount  of  matter  in  existence  always  remains  the  same,  and 
matter,  therefore,  cannot  be  destroyed  any  more  than  created,  the  amount 
of  its  inherent  force  to  produce  motion,  and  the  effects  produced  by  it  at 
any  moment  must  also  remain  the  same.     Applying  this  to  the  forces 
producing  motion,  it  follows  from  this  and  what  has  been  said  of  Inertia, 
that  motion  can  no  more  be  destroyed  or  created  than  matter  itself;  and  as 
all  matter  is  now  in  constant  motion,  motion  must  be  coeval  with  matter. 

23.  The  forces  and  properties  of  which  we  have  spoken  so  far  (Cohesion, 
Adhesion,  Gravity,  Impenetrability,  Impact  and  Inertia),  are  the  main 
causes  due  to  ponderable  matter  itself,  on  which  depends  its  position  in 
space.     There  are  yet  other  attractions  and  repulsions  between  the  atoms 
and  masses  of  ponderable  matter,  such  as  the  expansion  by  heat,  the 
attractions  and  repulsions  by  electricity,  &c. ;  but  these  seem  to  be  con- 
nected with  or  imparted  to  it,  by  certain  states  or  motions  of  the  ether 
between  its  atoms,  and  the  causes  of  which  are  designated  as  light,  heat, 
magnetism  and  electricity.     They  will,  therefore,  be  treated  of  separately 
in  connection  with  the  ether.     We  thus  obtain  two   parts   of  Physics 
proper,  Physics  of  Ponderable  matter,  or  Mechanical  Physics ;  and  Physics 
of  Imponderable  matter,  or  Ethereal  Physics,  which  treats  of  the  ether,  and 
the  influences  it  exercises  on  ponderable  matter.     Physics  of  Ponderable 

15 


16 


BOYE'S   INANIMATE   MATTER. 


matter  we  again  subdivide  iiito  three  sections;  Physics  of  Solids,  or 
Steoretics;  of  Liquids,  or  Hydraulics;  and  of  Gases,  or  Pneumatics. 
Physics  of  Imponderable  matter  is  sub-divided  into  four  sections;  Physics 
of  Light,  or  Photics,  or  Optics;  Physics  of  Heat,  or  Thermics;  Physics 
of  Magnetism,  or  Magnetics;  Physics  of  Electricity,  or  Electrics.  The 
following  table  will  exhibit  the  respective  divisions  and  subdivisions  of  the 
Natural  Sciences. 


'  Physics  of 

Solids,  or 

Stereotics. 

Physics  of 

• 

Ponderable 

Physics  of 

Matter. 

Liquids,  or 

(Mechanical 

Hydraulics. 

Physics.) 

Physics  of 

Gases,  or 

Pneumatics.  " 

f  Physics 

proper,  or 

Natural 
Philosophy. 

r  Physics  of 
Light,  or 

Photics,  or 

Optics. 

Physics  of 
Inanimate 
Matter. 

Physics  of 
Imponderable 
Matter. 

Physics  of 
Heat,  or 
Thermics. 

(General 
•Physics.) 

(Ethereal  or 
Imponderable 

Physics.) 

Physics  of 
Magnetism,  or 
Magnetics. 

Physics  of 

PHYSICAL  OB 

NATURAL 
SCIENCES. 
(Physics  in  its 

Chemistry. 
(Atomic  or 
Chemical 
Physics.) 

Electricity,  or 
Electrics. 

widest  sense.) 

Physics  of 
Animate 
Matter,  or  % 
Physiology. 
(Special 
L  Physics.) 


3> 


16 


PART  I. 


PHYSICS  OF  PONDERABLE  MATTER. 

THOUGH  in  a  systematic  point  of  view  it  would  be  better  to  treat  first 
of  solids,  still  as  it  practically  is  more  important,  first  to  have  a  know- 
ledge of  the  physical  properties  of  gases,  we  shall  begin  with  these. 

SECTION  I. 
PNEUMATICS,  OR  PHYSICS  OF  GASES. 

The  word  Pneumatics  is  derived  from  a  Greek  word  -KVWIIO.  (pneuma), 
signifying  air. 

Properties  of  gases  depending  on  Cohesion. 

24.  We  have  seen  that  whenever  the  repulsive  force  between  the  atoms 
preponderates  over  the   attractive,   matter  assumes  the  state   called  the 
gaseous  or  aeriform.     Gases,  therefore,  not  only  possess  fluidity  like  liquids, 
that  is,  they  offer  but  a  slight  resistance  to  the  moving  of  their  particles 
among  themselves,  but  their  atoms  have  also  a  constant  tendency  to  recede 
from  each  other,  and  therefore  to  extend  themselves  over  space,  until  limited 
or  confined  by  some  outer  boundary,  or  restrained  by  some  counteracting 
force.     This  property  is  called  Expansibility,  and  constitutes  the  main 
difference  between  gases  and  other  states  of  matter. 

25.  Nature  has  placed  us  in  an  ocean  of  gases  called  the  Atmosphere, 
which  forms  the  uppermost  portion  of  the  whole  earth.     Thus  circum- 
stanced, we  are  apt  to  feel  less  conscious  of  their  material  existence  and  to 
overlook  the  fact,  that  they  form  the  medium,  through  which  we  generally 
receive  the  impressions  on  our  senses  from  other  bodies.     Thus  when  we 
hear  a  sound  caused  by  the  vibrations  of  a  solid,  it  is  not  these  latter  that 
act  on  our  ears,  but  the  vibrations  of  the  air  produced  by  them.     And  if, 
in  the  same  manner,  on  account  of  the  extreme  fluidity  and  tenuity  of  the 

B  17 


18  BOYE'S  INANIMATE   MATTER. 

atmospheric  gases,  they  under  ordinary  circumstances  are  not  perceived, 
we  may  easily  render  air  as  tangible  as  a  solid  or  liquid  by  allowing  it  to 
impinge  against  any  part  of  our  body;  for  instance,  by  blowing  on  it. 
And  even  to  our  eye-sight  air  is  as  visible  as  any  other  kind  of  transparent 
matter;  we  have  colored  gases,  and  a  bubble  of  a  colorless  gas  is  as 
visible  in  water,  as  a  drop  of  water  is  in  air.  Their  effect  on  the  senses  of 
smell  and  taste  is  also  familiar.  It  will  also  be  shown,  hereafter,  that  we 
are  capable  of  weighing  gases  like  any  other  forms  of  ponderable  matter. 

26.  Atmospheric  air  is,  however,  not  one  kind  of  gas,  but  a  mechanical 
mixture  of  four  different  gases.   Oxygen,  about  -J  by  vol.,  and  Nitrogen,  about 
-J,  or  more  accurately,  in  the  relative  proportion  to  each  other  of  20.8  ox., 
to  79.2  nitr.,  form  the  main  portions  of  it.     Besides  these  it  contains  small 
but  varying  quantities  of  Carbonic  acid  (about  J  per  mille),  and  Yapor  of 
water  (£  to  2  per  cent.). 

27.  On  account  of  its  expansibility  it  might  be  supposed,  that  the  at- 
mosphere surrounding  the  earth  would   extend  itself  infinitely  far  into 
space.     This  is,  however,  not  the  case.     We  can  prove  from  the  property 
of  refraction,  which  the  atmosphere  possesses,  or  that  of  bending  the  light  from 
its  straight  path,  when  penetrating  in   an  oblique  direction  through  its 
strata  of  different  densities,  that  it  does  not  extend  sensibly  beyond  the 
height  of  45  miles.     It  is  therefore  probable,  that  as  the  rarefaction  of  the 
atmospheric  gases  increases  with  the  distance  from  the  earth,  their  expan- 
sibility also  becomes  less,  and  is  at  last  overcome  by  gravity,  drawing 
them  toward  the  earth,  so  that  where  these  two  forces  are  equal,  they 
will  assume  a  definite  limit.     This  is  confirmed  by  the  experiments  of 
Faraday,  according  to  which  the  vapors  of  mercury  enclosed  in  a  tall  jar, 
only  rise  to  a  certain  height,  presenting  an  upper  level  surface.     If  this 
be  correct,  the  different  gases  of  which  the  atmosphere  is  formed,  ought  to 
assume  each  a  separate  level  at  different  heights  from  the  surface  of  the 
earth,  according  to  their  different  densities.     This  might,  however,  be  pre- 
vented by  the  commotion  caused  by  currents. 

28.  The  expansibility  of  gases  affords  us  the  means  of  removing  them 
from  any  containing  vessel,  or  of  rarefying  them  to  any  extent.     Appa- 
ratus constructed  for  this  purpose  are  called  exhausting  air-pumps.     In 
the  simplest  form  an  exhausting   air-pump   consists  of  a  single  hollow 
cylinder,  generally  of  brass  (see  a  Jigs.  1  and  2),  called  the  barrel,  and 
having  the  inside  ground  perfectly  true,  so  that  a  short  solid  cylinder  b, 
called  the  piston,  may  be  moved  in  it  perfectly  air-tight  by  the  aid  of  the 
piston-rod   c,    furnished   for   this   purpose   with   a   handle   d.      At   the 
bottom  of  the  barrel  is  an  orifice,  which  forms  the  beginning  of  a  passage, 

1  and  efig.  2),  which  at  its  other  extremity  is  furnished  with  a 
18 


PNEUMATICS. 
Fig.  1. 


19 


Fig.  2. 


Fig.  3. 


screw  /,  by  which  it  can  be  attached  to  any  vessel  or  receiver  h,  from 
which  it  may  be  desirable  to  exhaust  the  air.  Across  this  passage  ef, 
as  near  as  possible  to  the  barrel,  is  inserted  a  conical  piece 
of  metal  g  figs.  1  and  2,  and  represented  separately  by 
fig.  3,  called  the  plug,  fitting  across  the  passage  in  a 
corresponding  conical  hollow,  so  as  to  be  movable  round  its 
axis,  which  is  at  right  angles  to  the  passage;  the  whole, 
the  passage  with  its  conical  hollow  and  the  plug,  con- 
stituting a  stop-cock.  The  plug  of  a  stop-cock  has  always 
one  perforation  through  it,  which  in  one  position  forms  a 
continuation  of  the  passage;  but  when  the  plug  is  turned  90  degrees 
round  its  axis,  so  as  to  have  the  perforation  at  right  angles  to  the  passage, 
this  is  interrupted.  The  stop-cock  used  in  this  case  is  what  is  termed 
a  two-ways  stop-cock,  having  two  perforations,  see  fig.  3,  the  usual  one 
*  to  close  and  interrupt  the  passage  e  f  between  the  barrel  and  the 
receiver  h,  to  which  the  air-pump  is  attached,  see  fig.  1,  and  a  second 
one  k  fig.  3,  which  when  the  first  perforation  is  at  right  angles  to 
the  passage,  see  fig.  2,  forms  at  first  a  continuation  of  it,  but  then  turns 
so  as  to  run  parallel  with  the  axis  of  the  plug,  and  terminates  outward 
into  the  atmosphere,  thus  establishing  a  communication  between  the  barrel 
and  the  outer  air,  when  the  communication  with  the  receiver  is  shut  off, 
as  seen  in  fig.  2,  which,  however,  is  interrupted  when  the  communication 
with  the  receiver  is  open,  as  seen  in  fig.  1. 

29.  If  now  after  having  attached  the  air-pump  to  any  vessel  or  receiver 

h,  from  which  we   intend  to  exhaust  the  air,   and   having  turned   the 

19 


20  BOYE'S  INANIMATE  MATTER. 

stop-cock  g,  so  as  to  establish  a  communication  between  it  and  the  barrel, 
see  fig.  1,  we  draw  out  the  piston  as  represented  in  fig.  2,  the  air 
in  the  receiver  will  expand  and  fill  both  the  receiver  and  the  barrel.  The 
stop-cock  is  then  turned  so  as  to  shut  off  the  communication  between  the 
receiver  and  the  barrel,  and  to  open  it  between  the  barrel  and  the  outer 
atmosphere,  as  represented  in  fig.  2,  and  the  piston  pushed  in  to  the 
bottom  of  the  barrel,  by  which  the  air  in  the  barrel  is  expelled  into  the 
atmosphere.  If  then  again  by  turning  the  stop-cock,  the  communication 
be  interrupted  between  the  barrel  and  the  atmosphere,  and  opened  between 
the  barrel  and  the  receiver,  and  the  piston  drawn  out,  and  the  same  process 
repeated,  a  portion  of  air  will  by  every  outward  stroke  of  the  piston  enter 
from  the  receiver  into  the  barrel,  and  by  the  next  inward  stroke  be  ex- 
pelled into  the  atmosphere.  This  might  thus  be  continued  as  long  as  the 
remaining  air  retains  its  expansibility,  though  a  last  portion,  however 
small,  would  always  remain  behind.  Practically,  however,  it  is  not  pos- 
sible to  carry  the  exhaustion  this  far  j  for,  however  near  the  plug  of  the 
stop-cock  be  placed  to  the  barrel,  a  small  space  will  always  remain  between 
it  and  the  bottom  of  the  latter,  called  the  Injurious  Space,  into  which  the 
piston  cannot  enter.  After  the  piston  has  been  pushed  to  the  bottom  to 
expel  the  air  in  the  barrel  into  the  atmosphere,  this  space  will  always  re- 
main filled  with  air  of  the  same  density  as  the  atmosphere.  If  this  air 
which  thus  remains  in  the  injurious  space,  by  expanding  over  the  barrel 
when  the  piston  is  again  drawn  out,  be  yet  of  the  same  density  as  the 
remaining  air  in  the  receiver,  none  of  the  latter  can  enter  into  the  barrel, 
when  the  communication  between  them  is  established,  and  thus  all  further 
exhaustion  becomes  impossible.  Besides  this,  such  apparatus  are  often  apt, 
from  imperfect  make,  to  admit  small  portions  of  air  by  leakage. 

30.  Stop-cock  pumps  have  the  inconvenience,  that  the  stop-cock  must 
be  turned  at  every  stroke.  This  may  be  performed  by  mechanical  contri- 
vances connecting  it  with  the  motion  of  the  piston-rod,  and  they  then  con- 

Fig.  4. 


. 

PNEUMATICS.  21 

stitute  very  superior  pumps.  It  is,  however,  more  convenient  and  less 
expensive  to  substitute  pneumatic  valves,  which  are  self-acting.  Such 
valves  are  generally  constructed  of  a  strip  of  oil-silk,  see  v  and  v1-  fig.  4, 
and  v  fig.  5,  fastened  by  its  two  extremities,  so  as  to  lay  close  over  the 
orifice  by  which  the  passage  terminates,  or  when  the  valve 
is  placed  in  the  passage  itself,  the  latter  is  made  to  ter- 
minate by  an  orifice  s  in  a  projection,  over  which  the  oil- 
silk  v  is  tied  or  otherwise  fastened,  as  shown  by  fig.  5, 
which  represents  separately  the  valve-piece  screwed  into 
the  piston  of  fig.  4.  Such  valves  will  then  allow  the 
air  to  pass  in  the  one  direction  between  it  and  the  orifice,  but  as  soon 
as  the  air  presses  in  the  opposite  direction,  the  oil-silk  is  forced  close 
against  the  orifice  and  prevents  the  air  from  passing  in  that  direction". 
Instead  of  the  two-ways  stop-cock,  two  such  valves  are  substituted,  see  v 
and  v*fig.  4.  One  v  is  placed  in  the  bottom  of  the  barrel  over  the  orifice 
--„  of  the  passage  leading  to  it  from  the  receiver,  so  as  to  allow  the  air  to 
pass  from  the  receiver  into  the  barrel  but  not  back  again.  The  other 
valve  v1  is  placed  in  a  passage  through  the  piston,  permitting  the  air  to 
pass  out  through  the  piston  from  the  barrel  into  the  atmosphere,  but  not 
back  again.  It  will  thus  be  evident,  that  every  time  the  piston  is  drawn 
out,  the  air  in  the  receiver  is  allowed  to  pass  through  the  valve  v  into  the 
barrel,  the  valve  v1  in  the  piston  remaining  closed.  When,  on  the  con- 
trary, the  piston  is  pushed  in,  the  valve  v  between  the  barrel  and  the 
receiver  closes,  and  the  air  in  the  barrel  is  expelled  through  the  valve 
v1  in  the  piston. 

31.  As  it  is  often  desirable  to  place  in  the  exhausted  vessel  different 
objects  or  apparatus,  it  becomes   necessary  to  have  pneumatic  receivers 
with  large  mouths  or  openings.     They  are  then  generally  made  bell-shaped 
or  cylindrical,  closed  at  the  top,  see  hfig.  4,  but  open  at  the  other  ex- 
tremity, the  edge  of  which  is  ground  true,  so  as  to  fit  air-tight  on  a  brass 
or  glass  plate  p,  also  ground  perfectly  plane,  and  having  an  opening  in 
its  centre  leading  to  a  passage  furnished  at  its  other  extremity  with  a  stop- 
cock and  a  screw,  to  which  the  air-pump  may  be  attached.     Any  object 
may  then  be  placed  on  the  plate,  after  which  the  bell  jar,  having  had  its 
edges  greased,  is  inverted  over  it  and  pressed  with  the  edges  against  the 
plate,  so  as  to  form  a  perfectly  air-tight  joint. 

32.  A  single  barrelled  air-pump,  or  Syringe,  as  called  when  small  and 
worked  by  hand,   always  acts  unequally,  requiring,  on  account  of  the 
atmospheric  pressure  on  the  piston  (see  further  on),  much  more  force  to 
move  the  latter  out  than  back  again.     To  avoid  this  and  also  to  expedite 

the  exhaustion,  which  is  a  tedious  process  when  the  capacity  of  the  barrel 

21 


22 


BOYE'S   INANIMATE  MATTER. 


is  small  in  proportion  to  that  of  the  receiver,  double-barrelled  air-pumps 
are  constructed,  see  fig.  6.  These  consist  of  two  complete  air-pumps, 
each  barrel  a  and  b  having  its  piston  and  two  valves,  one  in  the  piston 

Fig.  6. 


and  the  other  at  the  bottom  of  the  barrel  in  the  passage  to  the  receiver. 
But  these  two  passages  unite  into  one  leading  to  the  receiver  7i,  ter- 
minating at  the  plate.  The  piston-rods  are  furnished  with  teeth,  so  as  to 
form  racks  c  c,  which  are  moved  by  a  small  cog-wheel  or  pinion  c?,  to 
the  axis  of  which  is  attached  a  two  armed  lever  e,  with  handles  f. 
By  moving  the  lever  and  consequently  turning  the  pinion  in  alternate 
directions,  one  piston  is  always  moved  up,  while  the  other  is  moved  down, 
thus,  while  the  one  barrel  is  exhausting  the  receiver,  the  other  is  dis- 
arging  air  into  the  atmosphere. 

§13.  ^foratead  of  a  double-barrelled  air-pump,  a  single-barrelled  but  double- 
"ng  may  be  used,  as  represented  in  fig.  7.  In  this  case  the  cover  of  the 
barrel  must  be  air-tight,  and  the  piston-rod  made  to  slide  air-tight  through 
it  by  means  of  a  stuffing  box  or  packing  screw.  This  consists  of  a  hollow 
cylinder  s  fig.  7,  made  in  the  cover  round  the  piston-rod  c,  where  it 
passes  through  it.  Into  this  stuffing  box,  the  bottom  of  which  has  a  per- 
foration, merely  sufficient  to  let  the  piston-rod  pass  through  it  without 
friction,  the  stuffing  or  packing  is  introduced,  consisting  of  oiled  hemp  or  tow, 

or  circular  pieces  of  leather  (washers  or  collars),  with  perforations  through  their 

22 


PNEUMATICS. 

Fig.  7. 


23 


middle,  barely  sufficient  to  allow  the  piston-rod  to  be  pushed  through  them. 
A  screw  stopper  /,  also  perforated  through  its  middle,  but  so  as  to  allow 
the  rod  to  pass  easily  through  it,  is  then  screwed  down  into  the  stuffing 
box,  so  as  to  force  the  hemp  or  leather  washers  against  the  piston-rod,  so 
that  the  latter  may  slide  air-tight  through  it.  The  barrel  has  four  valves, 
u  and  u± — v  and  v±  which  in  this  case,  as  always  when  the  pumps  are 
large  and  subject  to  constant  wear,  are  made  of  metal,  and  have  then 
generally  a  conical  shape,  fitting  air-tight  in  a  corresponding  conical 
aperture  called  the  valve-seat.  By  any  pressure  from  the  one  side,  these 
valves  are  forced  from  their  seat,  while  pressure  from  the  other  side  will 
force  them  back  again.  To  restrain  their  motion  and  secure  their  easy 
return  into  their  seat,  they  are,  in  most  cases,  furnished  with  a  stem, 
which  slides  in  a  cross-piece  or  guide.  As  the  air  when  rarified  would  soon 
become  incapable  of  opening,  by  its  expansibility,  such  valves,  they  must, 
for  exhaustion,  as  in  the  present  case,  be  moved  by  some  mechanical  con- 
trivance. Of  the  above  four  valves,  two,  v  and  v±  open  inward  to  admit 
the  air  from  the  receiver  h  into  the  barrel,  and  are  worked  by  a  valve-rod 
o  sliding  air-tight  through  the  piston  b.  The  two  others,  u  and  u±  open 
outward  to  let  the  air  out  from  the  barrel  into  the  atmosphere,  and  are 
held  in  their  places  by  spiral  springs.  In  order  to  secure  their  opening  to 
expel  the  air,  they  have  a  short  stem  projecting  into  the  barrel,  against 
which  the  piston  strikes,  when  it  arrives  near  either  end.  Leading  from 

the  valves,  v  and  v±  which  open  inward,  are  two  passages,   L  and  tu 

23 


BOYE'S   INANIMATE   MATTER. 

Fig.  8. 


uniting  into  one  t  leading  into  the  receiver  h  through  the  plate  p. 
Being  made  of  lead,  and  therefore  flexible,  the  tube  t  may  easily  be  con- 
nected or  disconnected  with  the  plate  by  a  knob  and  gallows-screw  joint  m. 
It  will  easily  be  seen  that  by  each  stroke  the  piston  must,  on  the  one  side, 
draw  air  in  from  the  receiver,  while  on  its  other  side  it  expels  the  air  from 
the  barrel  into  the  atmosphere.  Fig.  8  gives  a  full  view  of  a  pump  of 
this  kind,  constructed  by  Dr.  Hare,  and  used  by  him  for  many  years  in 
his  Laboratory.  It  has  two  additional  passages  leading  from  the  valves  u 
and  u±  uniting  also  into  one,  open  to  the  atmosphere  at  n.  These,  how- 
ever, are  not  necessary  when  used  only  for  exhaustion. 

34.  Fig.  9  exhibits  another  efficient  single-barrelled  but  single-acting 
exhausting  air-pump,  of  Boston  manufacture,  often  met  with,  and  known 
as  an  Improved  (  Leslie'  Air-pump.  The  piston-rod  c  passes  air-tight 
through  a  stuffing-box  s,  in  the  top  of  the  barrel  a,  its  end  sliding  in  a 
cross-piece  or  guide  d,  to  keep  it  perpendicular  during  its  motion;  t — t  is 


PNEUMATICS. 
Fig.  9. 


25 


the  tube  forming  the  passage  from  the  barrel  to  the  plate  p,  into  the 
receiver  h.  The  pump  has  two  valves,  one  in  the  piston,  opening  from 
the  receiver  towards  the  top  of  the  barrel,  the  other  in  the  top  of  the 
barrel  at  v,  opening  from  this  into  the  atmosphere.  These  valves  are  made 
of  circular  pieces  of  thin  calf-skin  soaked  in  oil  and  lard,  laying  close  over 
the  orifices,  that  at  v  being  fastened  on  one  side  by  the  cap-piece  screwed 
down  over  it.  From  this  latter  valve  the  tube  u,  which  is  removable, 
leads  into  a  cistern  f,  open  to  the  atmosphere  and  intended  as  a  recep- 
tacle for  the  oil,  as  also  for  ether,  or  other  volatile  liquids,  which  often 
have  to  be  removed  as  vapors  from  the  receiver  by  exhaustion  and  may 
condense  in  the  barrel  or  the  tube,  and  thus  be  forced  out  through  it. 

25  3 


26  BOYE'S  INANIMATE  MATTER. 

"When  the  piston  is  pushed  in,  the  valve  at  v  prevents  the  air  from  entering 
into  the  barrel,  and  a  vacuum  is  formed  in  the  barrel  above  the  piston, 
into  which  the  air  enters,  by  its  expansibility,  from  the  receiver  and  barrel 
below  the  piston  through  the  valve  in  the  latter;  when  the  piston  is  raised, 
the  air  above  the  piston  cannot  return  through  the  valve  in  it,  and  is 
forced  out  through  the  valve  at  v  in  the  top  of  the  barrel,  while  the  barrel 
below  the  piston  is  again  filled  with  air  from  the  receiver,  following,  by 
its  expansibility,  the  piston  as  it  moves  out.  This  portion  of  air  in  the 
barrel  below  the  piston,  will  then  again  pass  through  the  valve  in  the 
piston  to  above  it,  when  this  is  again  pushed  in,  and  by  the  next  outward 
stroke  will  be  forced  out  as  before.  This  pump  has  the  advantage  over 
other  single-barrelled,  single-acting  air-pumps,  that  after  the  first  outward 
stroke  has  been  performed,  all  the  subsequent  ones  are,  as  in  the  double-acting, 
performed  through  the  greater  part  of  their  motion,  not  against  the  atmo- 
sphere, but  against  a  partial  vacuum,  until  the  piston  arrives  near  the  top, 
when  the  air  becomes  condensed  to  the  same  density  as  the  outer  atmo- 
sphere, and  of  course  the  last  effort  to  expel  it  through  the  valve  at  v,  must 
be  against  the  whole  atmospheric  pressure.  To  carry  the  exhaustion  to 
the  furthest  possible  limit,  the  tube  u,  may  be  removed  and  a  small  ex- 
hausting syringe  screwed  on,  by  which  a  vacuum  may  be  produced 
above  the  valve  at  v,  by  which  the  injurious  space  below  the  same  valve 
will  remain  filled  with  air  of  much  less  density  than  the  atmosphere, 
and  thus  have  less  effect  when  expanding  in  the  barrel  by  the  inward 
stroke  of  the  piston,  by  which  the  exhaustion  may  be  carried  much 
further. 

35.  The  amount  of  air  remaining  in  the  receiver  at  any  moment  during 
the  process  of  exhaustion,  or  the  degree  of  rarefaction,  may  be  calculated, 
assuming  that  no  leakage  takes  place,  by  knowing  the  relative  capacities 
of  the  receiver  and  the  barrel.  For  calling  the  former  R  and  the  latter  B, 
and  the  ordinary  density  of  the  air  D,  we  have  after  the  first  stroke,  that 
the  air  in  the  receiver  fills  both  the  receiver  and  the  barrel,  and  its  density 
after  the  first  stroke  Dt  must  therefore  be  to  its  former  density  D,  inversely 

11  T? 

as  the  spaces  occupied,  or  that  Dt  :  D  :  :  p         :  ^;  hence  Da  =  —  —  —  D. 

Jtv-j—  .15     xv  Jtx—  |—  Jt> 

After   the   second    stroke   we   get   in   the    same    manner    the    density 


D    = 


R 


, 
R+B  R-fB       R-f  B  R+B 


D  ==  (  -JL-Yl),  and  at  the  nth 
V/ 


stroke  Dn  ==   (  __  ^)  D.    Thus  if  the  barrel  have  |  the  capacity  of  the 


"R  0 

receiver,  we  have  R  =  9,  B  =  1,  and  -__  ,  =  „  and  the  density  or 

R-f-B         10 
26 


PNEUMATICS.  27 

quantity  remaining  in  the  receiver  at  the  3d  stroke,  =  Cri 


of  the  original  density  or  quantity. 

36.  The  rarefaction  at  any  time  is,  however,  generally  estimated  by  a 
barometer  guage  connected  with  the  receiver,  see  mfig.  6  and#,/#.  9, 
the  principle  of  which  will  be  explained  hereafter  under  pressure-guages. 

37.  Suction  by  the  mouth  depends  on  the  same  principle  as  exhaustion 
by  an  air-pump.     The  vessel  is  first  connected  by  the  lips  with  the  mouth, 
and  the  air  then  expelled  from  the  mouth  by  pressing  its  walls  close  to- 
gether.    A  vacuum  is  then  produced  in  the  mouth  by  withdrawing  the 
tongue  from  the  roof  of  the  mouth  without  admitting  any  air,  which  con- 
stitutes the  effort  of  sucking.     The  air  then  passes,  by  its  expansibility, 
from  the  vessel  into  the  mouth,  as  in  the  barrel  of  the  air-pump.     The 
communication  between  the  vessel  and  the  mouth  is  then  closed  by  using 

the  tongue  as  a  valve,  and  the  same  again  repeated.  4  r — 

38.  Besides  the  above  means  of  exhaustion  by  air-pumps,  a  partial 
vacuum  may  be  produced  by  the  increased  expansibility  of  gases  by  heat. 
Thus,  the  suction  of  an  ordinary  plain  cupping-glass  is  produced  by  ex- 
pelling a  portion  of  the  air  by  heat,  by  holding  it  with  the  mouth  down- 
ward over  a  spirit  lamp  or  a  piece  of  burning  paper,  and  then  quickly 
placing  it  on  the  skin.     Another  means  of  removing  atmospheric  air  from 
a  vessel  and  thus  producing  a  vacuum,  is,  by  the  introduction  of  a  volatile 
liquid  and  the  application  of  heat  to  it,  by  which  it  is   converted  into 
vapor,  which  will  expel  the  atmospheric  air.     By  then  closing  the  vessel 
and  allowing  the  vapors  to  condense,  a  vacuum  is    produced,  which  is 
entirely  free  from  atmospheric  air,  but  always  contains  more  or  less  vapor. 
Thus,  thermometer  bulbs,  and  other  vessels  with  very  narrow  mouths,  are 
filled  with  mercury  or  any  other  liquid,  by  first  expelling  a  portion  of  the 
atmospheric  air  by  heating  them  over  a  spirit-lamp^  and  then  inverting 
them  with  the  mouth  into  the  liquid.     When  the  .air  then  contracts,  a 
partial  vacuum  is  produced,  by  which  a  portion  of  the  liquid  is  forced 
up  into  it  by  the  atmospheric  pressure   (59).     They  are   then   again 
heated  till  the  liquid  inside  boils,  and  its  vapour  has  expelled  all  the  re- 
maining atmospheric  air,  when  they  are  again  inverted  with  the  mouth 
into  the  liquid,  by  which  they  become  entirely  filled  with  the  liquid  as 
soon  as  the  vapors  condense.      The  vacuum  in  the  cylinder  below   the 
piston  of  the  early  or  '  atmospheric'  steam-engine  of  Newcomen,  was  pro- 
duced by  the  expulsion  of  the  air  by  steam  from  a  boiler,  and  its  subse- 
quent condensation. 

S"""39C  Compressibility  and  Elasticity  of  gases.     From  the  nature  of  gases 
it  might  be  inferred,  that  the  atoms  are  not  so  close  together  as  in  liquids 

27 


28  BOYE'S  INANIMATE   MATTER. 

and  solids.  Indeed,  we  find  that  the  spaces  between  their  atoms  are 
capable  of  being  considerably  reduced  by  mechanical  pressure  and  their 
volume  in  consequence  diminished.  This  property  is  called  Compressi- 
bility. The  property  of  offering  to  the  compression  a  constantly  increasing 
resistance,  and  when  the  pressure  ceases,  of  again  resuming  their  former 
volume,  is  called  Elasticity.  Gases  thus"  possess  the  properties  of  Com- 
pressibility and  Elasticity  to  a  much  greater  extent  than  either  solids  or 
liquids. 

40.  This  is  also  the  reason  why  we  are  capable  of  forcing  a  considerable 
quantity  of  gas  into  a  comparatively  small  space.     Contrivances  for  this 
purpose  are  called  Forcing  or   Condensing  Air-pumps.     In  its  simplest 
form  the  Condensing  air-pump  is  identical  with  the  Exhausting  Syringe, 
see  figs.  1  and  2,  consisting  of  a  barrel  with  a  solid  piston,  and  furnished 
with  a  two-ways  stop-cock,  by  which  it  is  attached  to  the  receiver,  into 
which  the  air  is  to  be  condensed,  only  that  in  using  it,  the  order  of  turn- 
ing the  stop-cock  is  reversed.     For  if  the  piston  be  pushed  in,  while  the 
barrel  communicates  with  the  receiver,  it  is  easily  seen  that  the  air  con- 
tained in   the   barrel  must  be  forced   into  the  receiver.     If,  now,  the 
stop-cock  be  turned  so  as  to  shut  off  communication  with  the  receiver,  but 
to  establish  it  between   the  barrel   and  the    outer   atmospheric  air,  the 
latter  will  enter  and  fill  the  barrel  when  the  piston  is  again  drawn  out. 
By  repeating  the  same  process,  a  fresh  portion  of  air  is  by  every  inward 
stroke  introduced  into  the  receiver,   the  limit  being  dependent  on  the 
strength   of  the   apparatus  and  the   size  of  the   injurious   space  (29). 
For  it  will  easily  be  seen,  that  as  soon  as  the  air  admitted  into  the  barrel 
may  be  condensed  into  the  injurious  space,  without  acquiring  greater  den- 
sity than  the  air  in  the  receiver,  no  more  can  be  forced  into  it. 

41.  Instead  of  the  two-ways  stop-cock  we  may,  as  in  the  exhausting 
air-pump,  substitute  two  self-acting  valves  of  oil-silk,  see  fig.  10,  one  v 


Fig.  10. 

at  the  bottom  of  the  barrel  in  the  passage  leading  to  the  receiver,  and 
another  vt  in  a  passage  through  the  piston,  both,  however,  opening 
inward  as  represented  in  fig.  10.  The  valve  in  the  piston  may  be  dis- 
pensed with,  and  the  latter  remain  solid,  if  the  barrel  be  furnished  with  a 

28 


PNEUMATICS. 


29 


small  perforation  on  its  side,  at  a  distance  from  the  cover  just  sufficient  to 
be  cleared  by  the  piston  when  drawn  out,  for  the  admission  of  atmospheric 
air.  On  pushing  the  solid  piston  in,  the  air  thus  admitted  into  the  barrel 
is  confined  as  soon  as  the  piston  has  passed  the  orifice,  and  forced  into  the 
receiver,  and  so  on. 

42.  Where  larger  objects  are  to  be  placed  in  the  receiver,  the  latter 
must  be  furnished  with  a  wide  mouth,  see  fig.  12,  the  edge  of  which  is 

Fig.  11. 


ground  true  and  fitted  on  a  plate  as  for  exhaustion,  but  generally  with  the 
interposition  of  a  ring  or  washer  of  oiled  leather.  An  additional  contri- 
vance also  becomes  necessary,  to  keep  the  receiver  against  the  plate,  con- 
sisting of  two  uprights,  I  and  ?,  and  a  cross-piece  m,  which  can  be  screwed 
down  on  it,  as  otherwise  the  inner  pressure  of  the  air  would  force  them 
apart.  Such  receivers  should  also  be  made  as  much  as  possible  of  a 
spherical  form,  and,  if  of  glass,  very  thick,  as  much  greater  strength  is 

29 


30  BOYE'S  INANIMATE   MATTER. 

required  to  withstand  a  pressure  from  the  inside  than  from  the  outside, 
and  by  bursting  accidents  are  likely  to  occur. 

43.  Where  considerable  quantities  of  air  are  to  be  condensed,  the  pump 
may  be  made  double-acting  and  its  size  increased;  in  which  case  it  be- 
comes necessary  to  work  it  by  machinery.     When  high  degrees  of  con- 
densation are  required,  it  also  becomes   necessary  to  substitute  metallic 
valves  instead  of  those  of  oil-silk.     The  pump  jig.  7,  described   in  33? 
answers  admirably  for  condensing,  if  furnished  with  two  additional  passages 
leading  from  the  valves  u  and  «±,  as  represented  by  fig.  11,  which  two 
passages  unite  into  one,  terminating  in  a  knob  n,  so  that,  being  of  lead, 
and  therefore  flexible,  it  may  be  connected  by  a  gallows-screw    joint  m 
with  the  receiver  y^.  12,  into  which  the  air  is  to  be  condensed.     In  this 
use  of  the  pump  the  other  forked  tube  t,  fixed  over  the  valves,  opening  in- 
ward, must  of  course  be  left  open,  so  as  to  allow  the  atmospheric  air  free 
access  through  these  valves  into  the  barrel.     When  the  piston  is  moved, 
atmospheric  air  is  drawn  in  through  the  tube  t  on  the  one  side  of  the  piston, 
while  the  air  on  the  other  side  of  it  is  forced  into  the  receiver  through  the 
tube  n.     Such  pump  will  also  answer  for  transferring  and  condensing  any 
gas  different  from  atmospheric  air.     For  this  purpose  the  receiver  Jig.  12, 
into  which  the  gas  is  to  be  transferred  or  condensed,  is  first  exhausted  by 
being  connected  with  the  pump  by  the  tube  t.     It  is  then  to  be  connected 
with  the  pump  by  the  tube  n,  after  the  tube  t  has  been  connected  with  the 
receiver  containing  the  gas  to  be  transferred,  and  one  stroke  been  performed 
to  expel  the  atmospheric  air  from  the  barrel. 

44.  It  has  been  ascertained  by  accurate  experiments,  which  will  after- 
wards be  detailed,  that  the  volumes  which  a  gas  occupies  under  different 
pressures,  but  otherwise  similar  circumstances,  are  inversely  proportional 
to  the  pressures,  and  the  densities  of  the  gas,  therefore,  directly  propor- 
tional to  them.     This  law  is  called,  from  its  discoverer,  Mariotte's  law. 

iquefaction  of  gases.  In  regard  to  their  conduct  under  increased 
mres,  gases  differ  materially.  Some  of  them  obey  Mariotte's  law 
under  any  pressure  which  has  yet  been  applied  to  them,  and  are  there- 
fore called  permanent  gases.  Of  these  we  have  six;  Oxygen,  Hydrogen, 
Nitrogen,  Bin-oxide  of  Nitrogen,  Carbonic  Oxide  and  Light  Carburetted 
Hydrogen.  Others  conduct  themselves  in  a  similar  manner,  obeying 
Mariotte's  law,  only  until  the  pressure  has  been  increased  to  a  certain  point, 
when  they  suddenly  yield  and  are  converted  into  liquids.  These  are 
called  liguefiable,  sometimes  compressible,  or  condensable  gases,  the  latter, 
referring  mainly  to  the  fact,  that  this  same  effect  is  assisted  by  the  simul- 
taneous exposure  to  cold,  or  may  even  in  some  cases  be  produced  by  it 

alone.     Of  the  liquefiable  gases  a  certain  number  are  formed  from  sub- 

30 


PNEUMATICS.  31 

stances  existing,  under  ordinary  circumstances,  as  liquids  or  solids,  and 
when  filling  the  space  to  their  fullest  extent,  will  stand  no  increase  what- 
ever in  pressure  or  cold,  without  becoming  wholly  or  in  part  liquid. 
Such  gases  are  called  Vapors.  As  instances  of  liquefiable  gases  may  be 
mentioned  Sulphurous  acid,  liquefiable  at  a  pressure  of  about  5  atmo- 
spheres (1  atm.  =  151bs.  to  sq.  in.),  and  by  strong  cold  alone,  and  Car- 
bonic acid,  requiring  38  atmospheres  at  32°.  Of  vapors  may  be  men- 
tioned vapor  of  water  or  Steam. 

46.  It  is  supposed  that  all  gases  by  sufficient  pressure  would  become 
liquid,  but  even  should  this  not  be  the  case,  it  is  evident  that  no  pressure, 
however  great,  could  reduce  their  volume  to  nothing,  which  constitutes 
their  property  of  Impenetrability. 

47.  To  illustrate  the  compressibility  and  elasticity  of  the  atmospheric 
air,  fix  a  burning  taper  on  a  cork  floating  on  water.     Invert  a  large 
tumbler  or  jar  over  it,  and  depress  this  below  the  surface  of  the  water.     As 
the  depth  to  which  it  is  immersed  increases,  the  compressibility  of  the  air 
will  allow  the  water  to  ascend  to  a  greater  height  into  the  jar,  but  its 
elasticity  will  offer  a  constantly  increasing  resistance,  so  that  much  the 
greater  portion  of  the  jar  will  still  remain  filled  with  the  air  and  allow  the 
candle  to  continue  to  burn. 

48.  On  this  depends  the  action  of  the  diving-bell,  which  consists  of  an 
open  inverted  box  filled  with  air,  generally  made  of  cast-iron,  and  heavily 
loaded,  so  as  to  sink  when  let  down  into  the  water  by  a  rope,  and  fur- 
nished with  thick  glass  to  admit  light.     The  operator  is  supported  on  cross 
benches  near  the  bottom.     As  the  bell  is  lowered  to  a  greater  depth,  the 
pressure  of  the  water  becomes  greater,  and  the  air  in  consequence  more  and 
more  compressed,  so  that  the  water  ascends  higher  into  it.    To  prevent  the 
diver  becoming  thereby  partly  immersed  in  water,  and  to  replace  the  air, 
which  becomes  vitiated  by  the  respiration  and  the  burning  of  the  light 
sometimes  employed,  it  is  furnished  with  a  valve  and  hose,  through  which 
fresh  air  is  forced  in,  from  a  boat  above,  by  a  forcing  pump.     By  this 
means  it  soon  becomes  again  entirely  filled  with  air,  while  the  vitiated  air 
is  allowed  to  escape. 

49.  As.  .an  application  of  the  condensation  of  air  by  the  condensing  air- 
pump,  may  be  mentioned  the  air-gun,  of  which  the  essential  part  is  a 
strong  metallic  receiver,  into  which  atmospheric  air  is  compressed  to  a 
considerable  degree  by  a  condensing  syringe,  which  may  be  attached  to  it. 
Between  this  receiver  and  the  barrel  containing  the  ball,  is  a  valve,  which 
by  pulling  the  trigger  is  struck  open,  thereby  letting  out  a  portion  of  the 
confined  air,  which  propels  the  ball.     In  the  ordinary  air-gun  the  stock 

forms  the  receiver,  and  in  the  cane  air-gun  the  receiver  is  formed  out 

31 


32  BOYE'S   INANIMATE   MATTER. 

of  the   hollow  space   between  the  barrel  and  the  outer  tube   forming 
the  cane. 

50.  Besides  the  condensing  air-pump,  other  means  are  sometimes  re- 
sorted to  for  the  compression  of  gases.     Thus,  vapors  are  often  obtained 
in  a  compressed  state  by  the  introduction  of  a  volatile  liquid  into  a  con- 
fined space,  and  its  conversion  into  vapors  by  heat.     The  steam-boiler  is 
an  illustration  of  this.     The  high-pressure  steam-engine  may  be  considered 
as  a  single-barrelled,  double-acting  air-pump  attached  to  it,  the  barrel  being 
called  the  cylinder,  but  the  piston  of  which,  instead  of  condensing  the  gas 
by  its  motion,  is  itself  moved  by  the  elasticity  of  the  gas,  the  vapor  of 
water,  already  in  the  compressed  state  and  let  in  alternately  above  and  be- 
low the  piston. 

51.  Another  way  of  obtaining  gases  in  a  highly  compressed  state,  is  by 
generating  them  by  chemical  action  in  large  quantities  in  a  small  space. 
Fire-arms  may  be  considered  as  an  application  of  this,  the  mixture  em- 
ployed in  them  for  this  purpose  being  the  gunpowder.     Many  gases,  such 
as  carbonic  acid,  are  most  conveniently  liquefied  by  the  pressure  produced 
by  their  own  generation  in  an  appropriate  apparatus  (see  Chemistry  under 
Carbonic  acid). 

Properties  depending  on  Adhesion. 

I  N/  52.  The  repulsive  action  between  the  atoms  of  the  same  gas,  which 
causes  the  property  of  Expansibility,  we  do  not  find  to  exist  between  the 
V/  \  atoms  of  different  gases.  On  the  contrary,  the  atoms  of  one  gas  will 
allow  the  atoms  of  other  gases  to  push  themselves  between  them,  and  seem 
even  to  assist  this  action  by  an  attractive  force  toward  them  (Adhesion). 
Thus,  if  two  vessels,  h  and  cfig.  13,  separated  by  a  partition  p,  be  filled, 
the  upper  li  with  a  light  gas  as  hydrogen,  and  the  lower  c  by  a  heavy  gas 
as  carbonic  acid,  and  the  partition  between  them  be  withdrawn,  the  hydro- 
gen will  not  remain  on  top,  but  expand  and  spread  down- 
ward through  the  carbonic  acid;  and  in  the  same  manner 
will  the  carbonic  acid  rise  up,  spreading  through  the  hydro- 
gen,  till  they  both  are  evenly  diffused  through  the  whole 
P  z  space.  This  property  is  called  Diffusibility.  In  virtue  of 
this  property  one  gas  seems  hardly  to  offer  any  resistance 
to  the  expansibility  of  another,  and  gases  are  therefore 
not  capable  of  limiting  each  other,  or  of  maintaining  a 
distinct  boundary  between  themselves  (like  oil  and  water 
among  liquids). 

Fig.  13.  $\     53.  Diffusibility  of  gases  suffers  a  peculiar  modification, 
when  they  communicate  with  each  other  through  extremely  small  openings, 

32 


PNEUMATICS. 


83 


as  through  a  crack  in  a  glass,  or  through  a  porous  partition,  as  when 
formed  of  plaster  of  Paris,  unglazed  earthenware,  common  wood,  particu- 
larly when  cut  across  the  grain,  and  animal  membrane,  as  bladder,  skin, 
&c.  In  all  such  cases  the  lighter  gas  will  be  found  to  pass  through  such 
into  the  heavier,  faster  than  the  heavier  passes  in  the  opposite  direction  into 
the  lighter.  Thus,  if  in  fig.  13,  the  upper  vessel  h  be  filled  with  hydro- 
gen, and  the  lower  c  with  carbonic  acid,  and  the  partition  p  be  a  plate  of 
plaster  of  Paris,  it  will  be  found  that  the  hydrogen  will  pass  faster  into 
c,  than  the  carbonic  acid  into  h}  and  thus  a  partial  vacuum  is  produced  in 
the  vessel  h,  occupied  by  the  hydrogen,  and  a  condensation  in  c.  But  after 
some  time,  when  the  gases  become  thoroughly  diffused  through  each  other, 
equilibrium  is  again  restored  on  both  sides  of  the  partition.  This  may  be 

illustrated  by  the  diffusion  tube  b 
fig.  14,  which  is  a  glass  tube  open 
at  the  lower  end  and  closed  at  the 
upper  by  a  plug  a,  of  perfectly  dry 
plaster  of  Paris.  If  this  be  filled 
with  hydrogen  by  displacement  of 
the  atmospheric  air  (see  ),  so 
as  to  avoid  wetting  the  plaster  of 
Paris,  and  then  quickly  placed 
with  its  open  end  in  a  shallow 
vessel  dj  containing  water,  diffu- 
sion will  take  place  through  the 
Paris  plaster,  between  the  hydro- 
gen in  the  tube  and  the  atmo- 
spheric air  .outside,  and  the  hydro- 


Fig.  14. 


gen  passing  out  quicker  than  the  atmospheric  air  passes  in,  a  partiai 
vacuum  will  be  formed,  by  which  the  water  will  be  forced  up  in  the  tube 
to  c  by  the  atmospheric  pressure  (see  59),  several  inches  above  the  level 
outside.  But  after  some  time  it  again  falls  to  its  former  level.  This 
kind  of  diffusion,  particularly  when  taking  place  through  animal  or  vege- 
table membranes,  is  often  called  by  the  name  of  Endosmosis  and  Exos- 
mosis.  The  velocities  with  which  different  gases  diffuse  themselves,  have 
been  found  to  be,  under  otherwise  similar  circumstances,  inversely  pro- 
portional to  the  square  roots  of  their  densities  or  specific  gravities. 

54.  The  adhesion  of  gases  toward  Solids  is  quite  considerable,  so  that 
in  many  cases  it  causes  them  to  be  condensed  in  greater  or  less  quantities 
on  their  surface.  Thus,  it  is  found  that  ordinary  glass,  even  when  per- 
fectly dry  to  the  touch,  always  contains  a  thin  film  of  vapor  of  water  con- 
densed on  its  surface.  This  becomes  more  perceptible  when  its  surface 
C  33 


34 


BOYE'S  INANIMATE   MATTER. 


is  increased  by  pulverizing  it,  when  the  quantity  of  vapor  condensed  by  it 
may  be  so  great  as  to  amount  to  more  than  £  per  cent,  of  its  weight. 
The  same  is  the  case  with  most  other  pulverulent  or  porous  bodies,  such 
as  clay,  and  particularly  animal  and  vegetable  substances,  as  paper, 
wood,  hair,  membranes.  Such  water  is  called  hygroscopic  moisture  and 
is  found  to  vary  in  quantity  according  to  the  state  of  humidity  of  the 
atmosphere  ( 177  ),  and  interferes  materially  in  many  experiments  with 
the  accurate  determination  of  their  weight.  ,  Kecently  ignited  charcoal 
will  absorb  many  times  its  own  volume  of  different  gases,  such  as  oxygen, 
and  particularly  sulphuretted  hydrogen  and  other  similar  gases  or  vapors, 
which  are  the  cause  of  offensive  odors.  On  this  depends  its  preserving  and 
deodorizing  properties.  The  most  extraordinary  instance  of  such  condensa- 
tion of  gases  is  presented  by  platinum  towards  hydrogen  and  oxygen,  when  in 
porous  and  finely  divided  states,  in  which  it  is  called  platinum  sponge  and 
platinum  black,  the  latter  of  which  has  been  found  to  absorb  more  than 
250  times  its  own  volume  of  oxygen.  By  this  condensation  a  subsequent 
chemical  action  is  often  induced.  Thus,  oxygen  when  absorbed  by  -char- 
coal combines  after  some  time  with  it,  forming  carbonic  acid  in  its  pores; 
and  hydrogen  and  oxygen  when  absorbed  together  by  platinum  sponge 
unite  to  form  vapor  of  water,  so  that  platinum  sponge  when  held  before  a 
jet  of  hydrogen,  where  it  mixes  with  the  oxygen  of  the  atmosphere,  will 
become  heated  by  the  union  of  the  two  gases,  and  ignite  the,  jet  of  hydro- 
gen. On  this  depends  the  Platinum  Igniter, 
(Jig.  15),  which  is  an  apparatus  for  obtaining 
fire,  consisting  of  a  self-regulating  generator  of 
hydrogen  (see  ),  which  by  turning  up  the 
box  liy  opens  a  stop-cock  and  causes  the  hyd 
gen  to  issue  from  the  jet  e}  on  the  platin 
sponge  Ji±  and  thereby  to  become  ignited. 

55.  Towards  Liquids  also,  a  positive  a 
tion  or  adhesion  is  very  manifest,  by  which  the 
atoms  of  gases  are  drawn  in  between  the  atoms 
of  liquids,  which  constitutes  what  is  called  ab- 
sorption or  solution  of  gases  in  liquids.  Thus 
all  the  atmospheric  gases  dissolve  in  water  in 
small  quantities,  and  on  the  oxygen  thus  dissolved  (about  TJn  vol.  in  1 
vol.  of  the  water),  depend  all  gill-breathing  animals  for  their  respiration. 
Some  gases  dissolve  in  considerable  quantities  in  water,  as  carbonic  acid 
(1  vol),  and  sulphurous  acid  (50  vols).  It  is,  however,  often  difficult  to 
draw  the  line  between  mere  solution  or  absorption  and  chemical  combina- 
tion. Thus,  chlorohydric  acid  dissolves  in  water  to  the  amount  of  418  vols., 

34 


ro- 


Fig.  15. 


PNEUMATICS.  35 

and  aramoniacal  gas  to  the  amount  of  500  vols. ;  but  in  these  cases  a 
chemical  combination  with  the  water  takes  place  at  the  same  time. 

56.  When  a  gas  is  dissolved  in  a  liquid,  and  the  free  surface  of  this  solu- 
tion be  exposed  to,  or  brought  in  contact  with  another  gas,  or  be  separated  by 
a  porous  partition  from  it  or  from  a  solution  of  it  in  a  liquid,  diffusion  will, 
in  all  such  cases,  take  place  between  them.     It  is  by  such  diffusion  that 
by  respiration  an  exchange  takes  place,  through  the  membrane  of  the 
lung,  between  the  oxygen  of  the  air  and  the  carbonic  acid  dissolved  in 
the  blood:    and  that  in  gill-breathing  animals  an  exchange  is  effected, 
through  the  membrane  of  the  gill,  between  the  oxygen  dissolved  in  the 
water  and  the  carbonic  acid  dissolved  in  the  blood.     This  is  also  the  cause 
why,  when  gases  are  separated  by  liquids  in  which  they  are  more  or  less 
soluble,  an  exchange  of  them  always  takes  place  by  diffusion  through  the 
liquid.     This  is  not  only  the  case  when  a  gas  is  confined  by  a  very  thin 
film  of  liquid,  for  instance,  when  enclosed  in  a  soap-bubble;  but  even 
when  gases  are  kept  in  jars,  placed  with  their  mouth  in  water,  it  is  found, 
that  in  the  course   of  time  more  or  less   of  an   exchange  takes  place 
through  the  water  with  the  atmospheric  air  outside.     Thus,  if  the  gas  be 
hydrogen,   in  the  course  of  some   weeks,  some  of  it  will  have  escaped 
through  the  water,  while  a  perceptible  quantity  of  atmospheric  air  will 
have  found  its  way  through  the  water  into  the  hydrogen.     As  gases  are 
utterly  insoluble  in  mercury,  this  liquid  is  often  employed  for  confining 
them  more  perfectly,  and  answers  well  when  the  surfaces  of  the  glass  and 
the  mercury  are  perfectly  clean.     But  if  a  film  of  dust  cover  the  glass  or 
be  on  top  of  the  mercury,  when  immersing  the  mouth  of  the  vessel  into 
it,  so  as  to  prevent  perfect  contact  between  the  glass  and  the  mercury, 
diffusion  will  take  place  through  this  film. 

Properties  depending  on  Gravity. 

57.  Gases  are  subject  to  the  action  of  gravity,  and  they  are,  therefore, 
like  all  other  ponderable  matter,  attracted  by  the  earth  towards  its  centre, 
which  constitutes  their  weight.     To  prove  this,  attach  a  spherical  receiver 
furnished  with  a  stop-cock,  to  an  exhausting  air-pump,  and  having  removed 
the  air,  counterpoise  it  on  a  balance,  see  fig.  16,  so  as  to  produce  equili- 
brium.    Allow  then  the  atmospheric  air  to  fill  the  receiver  by  opening  the 
stop-cock.     It  will  be  found  that  the  receiver  now  weighs  more.     This 
gtin  is  due  to  the  weight  of  the  gases  which  now  fill  the  receiver.     By 
forcing  more  air  into  the  receiver  by  the  condensing  air-pump,  we  shall 
find  that  its  weight  is  still  further  increased.     By  accurate  experiments  it 
has  been  found,  that  100  cubic  inches  of  atmospheric  air,  freed  from  its 

35 


36 


BOYE'S   INANIMATE   MATTER. 


carbonic  acid  and  vapor  of  water,  at  30  inches  barometric  pressure  and  60° 
Fahrenheit,  weigh  exactly  30.82926  grains,  (or  at  32°  Fah.  32. 58685  grs). 
58.  Different  gases  have  different  weights  for  the  same  volume.  Thus, 
100  cubic  inches  of  oxygen  weigh  34.19  grains,  of  hydrogen  2.14  grains,  of 
carbonic  acid  47.14  grains.  By  the  density  or  specific  gravity  of  a  gas  we 
understand  the  number  which  expresses,  how  many  times  a  gas  is  heavier 

Fig.  16. 


than  the  same  volume  of  atmospheric  air,  which  is,  therefore,  the  standard 
of  comparison  and  its  specific  gravity  =  1.  To  obtain  the  specific  gravity 
of  a  gas,  we  first  fill  a  suitable  spherical  glass  receiver,  as  above,  with  atmo- 
spheric air,  freed  from  its  carbonic  acid  and  vapor  of  water  by  passing  it 
through  a  tube  filled  with  unslacked  lime,  and  ascertain  accurately  the 
weight  of  the  atmospheric  air  in  it.  We  then  again  exhaust  the  atmo- 
spheric air  and  fill  it  with  the  gas  (see  ),  at  the  same  temperature  and 
at  the  same  pressure,  and  ascertain  its  weight.  The  weight  of  the  gas 
divided  by  the  weight  of  the  atmospheric  air  will  then  give  us  its  specific 
gravity.  The  following  are  the  specific  gravities  of  some  of  the  different 


Atmospheric  air 1.0000          Nitrogen 0.97137 

Oxygen 1.1056          Carbonic  acid 1.529 

Hydrogen 0.06926        Vapor  of  Water 0.622 

To  avoid  fractions  the  specific  gravity  of  atmospheric  air  is  often  called 
1000  instead  of  1,  that  of  oxygen  then  becomes  1105,  hydrogen  69,  &c. 

*   59.  As  gases  possess  weight,  it  follows  that  the  surface  of  the  earth 

36 


PNEUMATICS. 


37 


must  sustain  a  considerable  pressure  from  the  weight  of  the  surrounding 
atmosphere  resting  on  it.  To  prove  this,  place  an  open  glass  tube  with 
one  of  its  extremities  in  water,  see  fig.  17,  and  remove  the  air  which  it  con- 
tains by  suction  with  the  mouth,  or  by  an  air-pump  attached  to  the  other 
end  a.  We  shall  find  that  as  the  air  is  removed,  the  pressure  of  the  atmo- 
sphere on  the  water  outside  the  tube  will  force  it  up  into  it.  On  re- 
admitting the  air  into  the  tube  the  water  will  again  fall  to  its  former  level. 
For  the  same  purpose  expel  the  air  from  a  tube  closed  at  one  end,  by  filling  it 
with  water  \  invert  it,  keeping  the  finger  on  the  open  end  to 
prevent  the  water  from  escaping,  and  introduce  this  end  into 
a  vessel  with  water.  On  removing  the  finger  the  water  does 
not  run  down,  but  the  tube  remains  filled  with  the  water  to 
the  top,  caused  by  the  pressure  of  the  atmosphere  on  the 
water  outside  of  it.  As  soon  as  the  air  be  again  in  any  way 
admitted  into  the  tube,  the  water  will  fall  as  before.  If  we 
perform  the  same  experiments  with  mercury  instead  of  water, 
and  use  a  tube  longer  than  30  inches,  we  shall  find,  that  on 
removing  the  air  from  the  inside,  the  pressure  of  the  atmo- 
sphere on  the  outside  is  not  capable  of  forcing  the  mercury 
up  to  the  top  of  the  tube ;  or  of  retaining  it  there,  if  closed  at  one 
end  and  filled  and  inverted  as  before,  but  only  at  the  perpendicular  height 
Fig.  18.  Fig.  19. 


37 


38  BOYE'S   INANIMATE   MATTER. 

of  about  30  inches  above  the  level  of  the  mercury  outside,  see  a  fig.  18,  and 
at  which  level,  therefore,  the  mercury  will  remain,  whatever  inclination 
we  give  the  tube,  as  represented  at  a±  alt  a±11  fig.  18.  That  it  still  is  the 
pressure  of  the  atmospheric  air  outside,  which  sustains  the  mercury  in  the 
tube,  may  be  further  proved  by  placing  the  whole  under  an  appropriate 
pneumatic  receiver,  see  fig.  ,19,  and  exhausting  the  air,  when  the  mercury 
in  the  tube  will  be  found  to  fall  as  the  air  is  withdrawn  from  outside  of  it; 
and  if  it  were  possible  to  remove  the  air  perfectly,  the  level  inside  and  out- 
side would  be  the  same  in  this  case,  as  when  the  atmosphere  is  both  in- 
side and  outside.  As  water  is  13.6  times  lighter  than  mercury,  the  atmo- 
spheric pressure  is  capable  of  forcing  it  up  to  a  height  13.6  times  greater 
than  that  of  the  mercury,  or  to  about  34  feet. 

o  60.  The  pressure  of  the  atmosphere  was  discovered  by  the  circumstance, 
that  some  Italian  pump-makers  had  in  vain  endeavored  to  raise  water  by 
a  suction-pump  to  a  greater  height  than  34  feet,  and  applied  to  Galileo  for 
the  reason.  Previously,  the  cause  of  water  rising  in  a  tube  under  such 
circumstances  had  been  ascribed  to  what  was  called  the  abhorrence  of 
nature  to  a  vacuum,  by  which  nature  always  endeavored  to  fill  it  up. 
Galileo  referred  the  subject  to  his  pupil  Torricelli,  who  at  once  suspected 
the  real  cause  to  be  the  pressure  of  the  atmosphere  consequent  to  its  weight, 
and  to  convince  himself  of  the  correctness  of  the  above  facts  in  regard  to 
water,  performed  (about  1643  A.  D.),  the  experiment  of  filling  a  tube  longer 
than  30  inches  with  mercury  and  inverting  it  in  a  cup  of  mercury.  Such 
apparatus  is  yet  called  after  him  a  Torricellian  tube.  The  real  proof, 
however,  of  the  mercury  in  the  tube  being  supported  by  pressure  from  the 
atmosphere,  was  obtained  by  Pascal  having  it  carried  up  a  high  mountain, 
by  which  the  air  underneath  became  incapable  of  pressing  on  the  mercury, 
and  this  therefore  gradually  fell  as  the  height  became  greater. 

61.  The  Torricellian  tube  furnishes  us  with  the  means  of  estimating  the 
pressure  of  the  atmosphere  on  the  surface  of  the  earth,  which  for  the 
greater  part,  though  not  entirely,  depends  on  the  weight  of  the  atmo- 
sphere. For  this  purpose  it  is  only  necessary  to  measure  accurately  the 
perpendicular  height  of  the  mercurial  column, — this  being  the  only  part 
of  it  which  is  sustained  by  the  atmosphere,  the  rest,  when  inclined  being 
supported  by  the  sides  of  the  tube.  This  height  will  be  found,  as  before 
stated,  to  be  about  30  inches.  The  pressure  of  the  atmosphere  on  Ifce 
surface  of  the  earth  is  therefore  equal  to  a  layer  of  mercury  all  over  it  30 
inches  in  height.  We  therefore  only  need  calculate  the  weight  of  a 
column  of  this  height  and  of  a  certain  base,  in  order  to  obtain  the  pressure 
of  the  atmosphere  on  an  area  equal  to  this  base.  We  thus  find,  that  a 
column  of  mercury,  which  has  the  height  of  30  inches  and  rests  on  a  base 

38 


PNEUMATICS.  39 

of  one  square  inch,  contains  30  cubic  inches  of  mercury  and  will  weigh 
14|  pounds,  which  is  therefore  the  amount  of  the  pressure  of  the  atmo- 
sphere on  every  square  inch  of  surface.  The  mercurial  column  in  the 
Torricellian  tube  does  not,  however,  always  remain  the  same,  but  is  found 
to  vary  in  the  same  place  at  different  times  about  3  inches.  The  pressure 
of  the  atmosphere  is,  consequently,  not  uniform,  but  varies  to  the  amount 
of  1 J  pound  on  the  square  inch.  In  most  calculations  it  is  considered  as 
being  equal  to  15  pounds  to  the  square  inch,  and  in  the  estimation  of 
pressures  this  is  considered  as  a  unit  under  the  name  of  one  Atmosphere, 
so  that  for  instance  a  pressure  of  3  atmospheres  means  a  pressure  of  45 
pounds  to  the  square  inch. 

62.  If  the  Torricellian  tube  be  prepared  with  care  so  as  to  expel  all  the 
atmospheric  air  and  moisture,  which  adhere  to  the  tube,  and  which  is  done 
by  boiling  the  mercury  in  it  before  inversion,  it  will  easily  be  seen  that 
the  vacuum  produced  above  the  mercury  by  the  subsequent  inversion, 
must  be  entirely  free  from  any  of  the  gases  of  the  atmosphere.  Hence, 
this  space  is  called  the  Torricellian  vacuum,  in  contradistinction  to  the 
vacuum  which  may  be  produced  by  an  air-pump.  At  the  temperature 
between  60°  and  80°  Fah.,  it  begins,  however,  to  contain  a  perceptible 
of  vapor  of  mercury. 


THE  BAROMETER. 

63.  As  the  pressure  of  the  atmosphere  varies,  it  becomes  important  to 
estimate  at  any  time  its  amount  with  accuracy.  Instruments  constructed 
for  this  purpose  are  called  Barometers,  from  ftapoq  (baros)  a  Greek  word 
signifying  weight,  and  perpov  (metron)  measure,  meaning  literally  mea- 
surer of  the  weight  of  the  air  (see  90  ).  In  the  ordinary  form  it  consists 
of  a  carefully  prepared  Torricellian  tube  (60),  inverted  in  a  very  small  cup 
or  cistern  containing  mercury,  and  furnished  with  an  accurate  scale,  by 
which  we  are  able  to  read  off  at  any  time  the  height  of  the  mercurial 
column  above  the  level  of  the  mercury  in  the  cup.*  This  is  called  the 
cup  or  cistern  barometer,  see  Jigs.  20  and  32.  In  order  to  fix  the  tube  to 

*  In  the  making  of  accurate  barometers  certain  precautions  are  necessary  in  the  filling 
of  the  tube.  By  keeping,  more  or  less  dust  always  finds  its  way  into  open  tubes.  Ba- 
rometer tubes  should  therefore,  if  practicable,  be  sealed  at  both  extremities  immediately 
after  they  have  been  drawn  at  the  glasshouse,  and  be  kept  in  this  state  till  ready  for  use, 
•when  one  end  is  cut  off.  Where  this  cannot  be  done,  it  may  become  necessary  to  wipe 
them  clean  inside  by  a  thin  copper  wire,  wrapped  over  with  dry  thread.  Should  it  be 
found  indispensable  to  clean  them  with  water,  this  is  best  removed  by  rinsing  with  strong 

39 


40 


BOYE'S  INANIMATE  MATTER. 


the  cup,  the  latter  may  be  furnished  with  a  cover  of  wood,  cut  across  the 
grain,  by  which  it  is  sufficiently  porous  to  let  the  atmospheric  pressure 
through  it,  without  allowing  the  mercury  to  be  spilled  out  of  the  cup,  and 
through  which  cover  the  tube  may  then  be  fixed  (fig.  32),  or  the  whole 
cistern  may  be  made  of  wood,  as  in  fig.  20,  the  top  being  in  one  piece 
with  it,  and  the  bottom  screwed  on  before  inverting  it.  Instead  of 

having  a  cup  attached  to  the 
tube,  the  tube  may  be  bent  at 
the  lower  extremity  so  as  to 
have  the  open  end  turned  up- 
ward, see  fig.  21,  in  which  case 
this  open  end  acts  as  the  cup, 
and  it  is  then  called  a  plain 
syphon  barometer,  or  if  the 
open  end  be  blown  into  a  bulb 
or  cup,  as  in  fig.  22,  it  is  called 
a  syphon  cup-barometer.  The 
whole  apparatus  is  then  fast- 
ened to  a  board,  Jigs.  21  and 
22,  or  enclosed  in  a  case  of 
wood  or  brass,  figs.  28  and  32, 
on  which  the  scale  is  fixed. 
The  whole  scale  is,  however., 
rarely  affixed  to  the  barometer, 
but  only  so  much  of  its  upper 
portion,  as  is  necessary  for  the 
intended  use;  on  ordinary  baro- 


Fig.  20. 


Fig.  21. 


Fig.  22. 


alcohol,  after  which  the  tube  is  dried  by  heating  it  on  the  outside  at  a  short  distance  from 
one  of  the  open  ends,  and  drawing  dry  air  through  it  by  suction  from  the  other  end.  As 
it  is  almost  impossible  to  remove  any  moisture  in  the  tube  after  it  has  been  sealed  at  one 
end,  the  greatest  care  should  be  taken  to  avoid  introducing  any  by  the  breath,  or  by  the 
flame  of  the  blowpipe  lamp.  The  sealing  should  therefore  be  done  by  drawing  the  tube 
out  at  such  a  distance  from  the  end  as  to  prevent  this.  Before  filling,  both  the  tube  and 
the  mercury  should  be  strongly  heated,  and  in  some  cases  it  may  even  be  necessary  to 
heat  the  mercury  to  boiling  after  its  introduction  into  the  tube.  The  mercury  employed 
should  be  purified.  This  is  generally  done  by  forcing  it  through  skin  and  by  digesting  it  in 
a  lukewarm  place  with  muriatic  or  diluted  sulphuric  acid.  For  standard  barometers  it 
should  be  distilled.  Distilled  mercury  is  apt  to  become  covered  with  a  black  film  at  the 
open  end,  but  this  is  prevented  by  subsequent  digestion  with  strong  muriatic  acid  and 
thorough  washing  with  water  to  remove  the  acid.  Syphon  barometers  are  filled  with  the 
mercury  as  high  up  as  practicable  before  bending  them,  after  which  the  filling  is  com- 
pleted through  the  open  end  by  suitable  manipulations.  Barometer  tubes  contracted  at  any 
point  to  capillary  dimensions  must  be  filled  in  the  same  manner,  as  thermometer  bulbs  (3S) 

40 


PNEUMATICS.  41 

meters  seldom  more  than  4  or  5  inches.  In  all  cases,  whether  cup  or 
syphon  barometer,  the  height  of  the  mercurial  column  is  measured  by  the  per- 
pendicular distance  from  the  level  of  the  mercury  in  the  open  part  to  the  top 
of  the  mercury  in  the  closed  end  of  the  tube. 

-  64.  All  barometers  have  the  inconvenience,  that  when  the  mercury  in 
the  upper  closed  end  of  the  tube  rises  or  falls  by  a  variation  in  the  pres- 
sure of  the  atmosphere,  a  portion  of  the  mercury  is  either  abstracted  from, 
or  added  to  the  mercury  in  the  open  part,  by  which  the  level  of  this  latter, 
which  forms  the  beginning  of  the  scale,  is  altered.  In  the  cup-barometer 
this  error  may  be  diminished  sufficiently  for  ordinary  purposes,  by  making 
the  upper  part  of  the  cup,  where  the  mercury  rises  and  falls,  see  g  fig.  20, 
of  a  considerably  larger  diameter  than  that  of  the  tube  at  the  upper  level 
of  the  mercury.  Thus,  if  the  diameter  of  the  cup  be  10  times  greater  than 
that  of  the  tube,  their  relative  contents,  which  are  proportional  to  the 
squares  of  their  diameters,  will  be  as  100  is  to  1,  and  therefore  a  fall  of 
one  inch  in  the  tube  will  only  raise  the  level  in  the  cup  y-J^  of  an  inch. 
Where,  however,  the  utmost  accuracy  is  required,  it  becomes  necessary  to 
avoid  this  error  altogether,  which  is  done,  either  by  making  the  scale 
movable  and  adjusting  its  lower  end  to  the  level  of  the  mercury  in  the 
cup,  or  by  furnishing  the  cup  with  a  movable  bottom  of  skin,  which  may  be 
raised  by  a  screw,  see  h  fig.  32,  by  which  the  mercury  may  always  be  ad- 
justed to  the  same  level.  This  level  is  sometimes  indicated  by  a  float  in 
the  mercury,  the  stem  of  which  passes  through  the  cover,  but  more  fre- 
quently, and  with  greater  reliance,  by  a  point  of  ivory  projecting  down 
from  the  cover  of  the  cup,  see  fig.  32  at  p,  the  cover  being  made  of  wood  cut 
across  the  grain,  so  as  to  allow  the  air  free  ingress  through  its  pores,  and 
the  sides  of  the  cistern  of  glass,  so  that  the  point  is  visible  through  it.  To 
adjust  the  level  of  the  mercury  in  such  cistern  before  making  an  observa? 
tion,  the  mercury  in  it  is  raised  by  the  screw  at  the'  bottom,  till  the  ivory 
point,  by  dipping  into  the  mercury,  forms  a  small  cavity  in  its  surface;  it 
is  then  lowered  till  this  cavity  just  disappears. 

65.  In  the  plain  syphon  barometer,  Jig.  23,  the  above  inconvenience 
may  be  avoided  by  having  the  bore  of  the  two  limbs  of  the  tube  of  exactly 
the  same  diameter  or  calibre.  It  will  then  be  seen,  that  when  the  mercury 
in  the  closed  end  rises,  for  instance,  i  inch,  it  must  fall  exactly  the  same 
amount,  or  £  inch,  in  the  open  end;  and  thus  the  difference  between  the 
two  levels  will  be  one  inch.  In  the  same  manner  all  changes  of  the  ba- 
rometer will  always  be  double  that  indicated  in  the  closed  end,  so  that  if 
the  barometer  be  correct  at  30  inches,  it  is  only  necessary  to  double  the 
value  of  the  other  divisions  of  the  scale,  that  is,  half  an  inch  above  is 
marked  31  inches,  and  half  an  inch  below,  29  inches,  and  so'  on.  As, 

41 


42 


BOYE'S  INANIMATE   MATTER. 


however,  it  is  extremely  difficult  to  obtain  the  bore  of  the  two  limbs 
Fig.  23.        Fig.  24.  Of  exactly  the  same  diameter,  any  uncertainty 

I 1       ffi          arising  from  a  variation  in  their  calibre,  may  be 

avoided  by  drawing  an  arbitrary  horizontal  line, 
see  a  Jig.  24,  between  the  upper  and  lower  level 
of  the  mercury,  and  furnishing  each  limb  with  a 
separate  scale,  which  two  scales,  s  and  s,  measure, 
the  one  the  distance  from  this  horizontal  line  to 
the  level  of  the  mercury  above  it  in  the  closed 
limb,  the  other  the  distance  from  this  same  line 
to  the  level  of  the  mercury  below  it  in  the  open 
limb,  which  two  measures  added  together  will 
give  the  true  height  of  the  whole  column. 

66.  A  great  object  in  a  good  barometer  is  to 
be  able  to  measure  with  accuracy  small  changes 
in  the  pressure  of  the  atmosphere.  But  on 
account  of  the  high  specific  gravity  of  the  mer- 
cury, being  nearly  11000  times  heavier  than 
atmospheric  air,  these  changes  are  only  indicated 
by  extremely  small  changes  in  the  mercurial 
column.  To  remedy  this  inconvenience,  so  as  to 
increase  the  actual  motion  or  sJiow  of  the  barome- 
ter, different  means  have  been  proposed.  As  the  first  of  these,  may  be  men- 
tioned the  substitution  of  a  specifically  lighter  liquid  instead  of  the  mercury, 
But  in  the  same  proportion  as  the  specific  gravity  of  the  liquid  becomes  less, 
the  barometer  becomes  longer  and  less  portable.  In  the  Koyal  Society 
of  London,  there  is  a  barometer  which  was  constructed  by  Daniell  with 
Water,  instead  of  mercury,  the  column  of  which  was  therefore  34  ft.  high, 
and  varied  by  the  changes  in  the  atmosphere  about  3  ft.,  so  as  to  be 
almost  constantly  in  a  state  of  motion.  But  besides  the  above  named  in- 
convenience from  its  size,  which  would  not  be  an  objection  for  stationary 
observatories,  all  such  liquids  are  liable,  if  volatile,  as  water,  to  evaporate 
from  the  open  end,  and  for  the  same  reason  to  form  a  vapor  in  the  vacuum 
at  the  closed  end,  which  varies  with  the  temperature,  and  of  which  an 
account  must  be  kept;  or,  if  not  volatile,  as  oil,  to  change  by  contact  with 
the  air  or  the  sides  of  the  tube. 

67.  The  mercury  being  thus  the  only  liquid,  which  can  be  employed  with 
advantage  in  the  construction  of  barometers,  it  has  been  attempted  to  pro- 
duce the  same  effect  of  increasing  its  show  by  attaching  certain  mechanical 
contrivances  to  the  mercurial  barometer. 

42 


PNEUMATICS. 


43 


68.  Thus,  in  the  Diagonal  or  Inclined  Plane  Barometer,  the  upper  closed 
portion  of  the  tube,  in  which  the  mercury  rises  and  falls,  instead  of  being 
perpendicular,  is  inclined  so  as  to  form  a  considerable  angle  with  the  per- 
pendicular.    As  the  changes  of  the  barometer  are  measured  by  the  perpen- 
dicular height,  it  is  evident,  that  the  mercury  in  order  to  arrive  at  the 
same  perpendicular  height,  must  travel  through  a  longer  distance  along 
the  inclined  part  of  the  tube,  and  thus  the  motion  of  the  barometer  is 
increased  in  the  proportion  of  the  hypothenuse  of  a  right  angled  triangle, 
to  its  perpendicular  side,  or  as  the  diagonal  of  a  rectangle,  to  the  same. 
But  as  only  the  perpendicular  part  of  the  mercury  on  the  inclined  portion 
is  supported  by  the  atmospheric  pressure,  the  rest  being  supported  by  the 
inclination  of  the  tube,  the  friction  of  the  mercury  against  the  sides  of  the 
tube  is  much  greater,  and  will  prevent  small  changes  in  the  pressure  of  the 
atmosphere  from  moving  the  mercury  until  they  become  larger,  when  they 
will  appear  in  the  above  increased  proportion.   Thus  the  small  changes,  which 
are  the  most  difficult  to  observe,  are  not  indicated  at  all  in  this  barometer. 

69.  Another  barometer  constructed  with  a  view  to  the  same  advantage, 
is  the  Wheel  Barometer  (Hooke's),  see  fig.  25,  which  consists  of  a  syphon 

Fig.  25.  barometer,  having  in  the  mercury  of  its  open  limb, 

an  iron  or  glass  float,  to  which  is  attached  a  string,  that 
passes  over  a  small  wheel  or  pulley  and  is  kept  extended 
by  a  small  weight  attached  to  the  other  end.  The  axis  of 
the  wheel  is  furnished  with  an  index,  which  traverses 
a  circular  scale.  It  will  easily  be  seen  that  when  the  level 
of  the  mercury  changes  in  the  open  end,  the  float  will 
follow  it  and  by  the  string  move  the  wheel,  and  its  index 
will  thus  pass  over  the  circular  scale,  the  length  of 
which  must  be  in  proportion  to  the  length  of  the  index/ 
The  graduations  on  the  scale  are  made  to  indicate  the 
corresponding  rise  and  fall  of  the  mercurial  column  in 
inches.  Though  as  regards  very  small  changes,  this 
barometer  is  liable  to  the  same  objections  as  the  for- 
mer, that  these  are  not  indicated  on  account  of  the 
friction  of  the  weight  and  the  pulley,  and  the  rigidity 
of  the  cord ;  still  for  ordinary  meteorological  purposes 
it  forms  both  a  cheap  and  an  handsome  instrument, 
and  is  therefore  often  met  with  in  parlors  and  studies, 
as  a  '  weather  glass/  As  regards  accuracy  they  are, 
however,  often  made  very  indifferently,  and  in  such 
cases  are  not  reliable  for  barometrical  observations. 


44 


BOYE'S   INANIMATE   MATTER. 


70.  A  third  barometer  of  this  kind  is  Huyghen's  Double-Barometer, 
Fig.  26.  Jig.  26.  It  is  a  syphon-barometer,  the  two  ends  of  which  are 
widened  where  the  mercury  rises  and  falls.  The  open  end 
terminates  in  a  long  open  capillary  tube.  The  mercury  of 
the  barometer  fills  half  of  the  wide  portion  of  the  open  end 
to  a,  but  the  other  half  of  it  and  part  of  the  capillary  tube, 
are  filled  with  colored  spirits  of  wine.  It  is  evident,  that  any 
change  in  the  level  of  the  mercury  by  the  pressure  of  the  atmos- 
phere, will  cause  a  certain  quantity  of  the  spirits  to  be  forced 
.30  into,  or  withdrawn  from,  the  capillary  tube,  and  thus  produce 
a  change  in  the  level  of  the  spirits  in  the  latter  so  much 
greater,  as  its  relative  capacity  is  less,  which  change  may  be 
magnified  to  any  desired  extent  by  diminishing  the  diameter 
of* the  capillary  tube.  It  has,  however,  been  found  that  the 
spirits  is  apt,  by  its  greater  adhesion  to  the  glass,  to  work  its 
way  between  the  mercury  and  the  tube  into  the  vacuum  at 
the  closed  end,  and  thus  render  it  liable  to  get  out  of  order. 
71.  All  these  contrivances  for  increasing  the  actual  motion 
or  show  of  the  mercurial  barometer  have  therefore  been 
abandoned  for  very  accurate  scientific  purposes,  and,  instead 
of  them,  increased  power  and  accuracy  of  observing  and 
measuring  have  been  substituted.  For  this  purpose  the  scale 
of  the  barometer  is  furnished  with  a  sight,  or  horizontal  line,  which  the 
observer  may  slide  along  the  tube  until,  by  looking  over  it,  he  may  bring 
the  top  of  the  mercury  on  the  same  horizontal  level  with  it,  and  thus  trans- 
fer the  level  of  the  mercury  to  the  exact  point  on  the  scale,  which  corres- 
ponds to  it.  On  account  of  the  difficulty  to  the  eye  to  count  small  divi- 
sions, the  scale  is  rarely  divided  into  smaller  parts  than  tenths  of  an  inch, 
or  at  most,  the  tenths  are  again  divided  into  halves,  or  T^ths.  As  on  this 
account  the  point  transferred  will  rarely  coincide  with  a  division  of  the 
scale,  a  vernier  is  attached  to  the  sight,  in  order  to  measure  the  exact  dis- 
tance of  the  point  from  the  nearest  division  of  the  scale. 
•"•"•72.  The  Vernier  see  v  vt  fig.  27,  is  a  short  scale  sliding  on  the  main  scale, 
the  use  of  which  therefore  is,  when  a  point  does  not  coincide  with  a  division 
of  the  main  scale,  to  measure  its  distance  from  this  division.  To  obtain  this 
distance,  one  of  the  extremities  of  the  vernier,  either  its  zero  or  its  highest 
number,  is  placed  at  the  point  in  question,  and  the  vernier  then  gives  its 
distance  from  the  last  counted  division  on  the  main  scale  by  a  fraction, 
which  has  for  its  numerator  the  number  of  that  division  of  the  vernier, 
which  coincides  with  a  division  on  the  main  scale,  and  for  its  denominator 
the  whole  number  of  divisions  of  the  vernier,  multiplied  by  the  denoniina- 

44 


PNEUMATICS. 


45 


27. 


tor  of  the  value  of  the  smallest  divisions  of  the  main  scale.  The  vernier 
is  always  fixed  in  such  manner  to  the  sight,  that  when  the  latter  is  brought 
on  a  level  with  the  top  of  the  mercury,  the  nearest  extremity  of  the  ver- 
nier (either  its  zero  or  its  highest  number)  is  made  to  indicate  the  exact 
point  on  the  main  scale,  which  corresponds  to  the  top  of  the  mercury.  If 
this  then  coincide  exactly  with  a  division  on  the  main  scale,  this  division 
is  counted  and  the  vernier  is  not  used.  But  if  the  extremity  of  the  ver- 
nier do  not  coincide  with  a  division  on  the  main  scale,  we  first  count  or 
read  off  the  height  to  the  nearest  lower  division  on  the  main  scale,  and  add 
to  this  the  distance  from  it  to  the  extremity  of  the  vernier,  which  distance 
is  obtained,  as  stated  before,  by  looking  along  the  vernier,  to  find  the  divi- 
sion on  it,  which  coincides  with  a  division  on  the  main  scale.  Thus,  let 
1 1  Jig.  27  represent  a  section  of  a  portion  of  the  tube  of  a  mercurial  baro- 
meter, with  its  scale  s  s  divided  into 
inches  and  tenths  of  inches,  a  the 
top  of  the  mercury  in  the  closed 
limb,  and  v  p  the  sight  transferring 
its  level  to  the  scale  s  s  at  p}  being 
also  the  zero-extremity  of  the  ver- 
nier v  v±.  It  is  evident  that  the 
nearest  lower  division  on  the  main 
scale  is  30.1  inch,  and  the  height 
to  the  point  p,  therefore,  30.1  inch 
-f-  the  distance  from  30.1  to  p. 
This  distance  is  then  given  by  the 
vernier  to  be  TJ^j  of  an  inch,  7 
being  the  number  of  the  division 
on  the  vernier,  which  coincides  with  . 
a  division  on  the  main  scale,  taking 
this  number  as  the  numerator,  while 
the  denominator  100  is  obtained  by 
taking  the  whole  number  of  divisions 
of  the  vernier,  10,  and  multiplying 
it  by  10,  the  number  which  is  the  de- 
nominator of  the  value  of  the  small- 
est division  of  the  main  scale  (J <jth 
of  an  inch.)  The  distance  from 
30.1  inch  top  thus  being  TJ7  = 
0.07  inch,  the  whole  height  of  the 
mercurial  column  must  of  course  be  30.1+0.07=30.17  inch;  so  that  hav- 

45 


46 


BOYE'S   INANIMATE   MATTER. 


Fig.  27. 


ing  read  off  on  the  main  scale  the  number  of  inches  and  tenths,  we  only 
have  to  add  the  number  on  the  vernier  at  the  coincidence  as  hundredths. 

73.  To  understand  this,  it  must 
be  stated,  that  the  vernier  always 
subdivides   the   smallest   divisions 
of  the  main  scale  in  as  many  parts 
as  it  has  itself  divisions.     This  is 
effected  by  taking  the  number,  into 
which  it  is  to  subdivide  the  smallest 
divisions  of  the  main  scale,  -f-  or 
—  1,  as  its  length,  and  dividing  this 
length  into  the  former  number  of 
equal  parts.     Thus  in  fig.  27  the 
smallest  divisions  of  the  main  scale 
are  tenths  of  inches,  which  the  ver- 
nier again   subdivides  into  tenths 
and  thereby  gives   hundredths  of 
inches.     It  is  therefore  constructed 
by  taking  eleven  (10-fl)  divisions 
of  the  main  scale  (j-Jths  inch)  as 
its  own  length,  and  dividing  this 
into  ten  equal  parts.    We  thus  have 
that  ten  divisions  of  the  vernier  are 
equal  to  eleven  of  the  scale,  or  10 
v  =  11  s,  therefore,  1  v  =  ly^s, 
or  that  each  division  of  the  vernier 
is  T\jth   larger  than   the  smallest 
division  of  the  main  scale,  and  as 
this   is   itself  one-tenth   inch,   each    division    of  the    vernier   must   be 
one  hundredth  of  an  inch  longer  than  the  smallest  division  of  the  scale. 
If  therefore  (see  fig.  27)  the  7th  division  of  the  vernier  coincides  with 
29.4  inch  on  the  main  scale,  the  6th  division  of  the  vernier  must  be 
one  hundredth  of  an  inch  above  29.5  (the  next  higher  division  on  the  main 
scale);  the  5th  be  two  hundredths  above  29.6;  the  4th  be  three  hundredths 
above  29.7;  the  3d  be  four  hundredths  above  29.8;  the  2d  be  five  hun- 
dredths above  29.9;  the  1st  be  six  hundredths  above  30.0;  and  0  or  the 
zero  point  be  seven  hundredths  of  an  inch  (7  being  the  number  at  the  coin- 
cidence) above  30.1  inch  on  the  main  scale.     The  distance  from  30.1  inch 
to  the  point  p  is  thus  indicated  by  the  vernier  to  be  0.07  inch,  as  stated 
above,  and  the  whole  height  of  the  mercurial  column  30.17  inches.     It  is 

46 


9- 


i 


-29  inch 


PNEUMATICS. 


47 


also  evident  from  the  same  fig.  27,  that  the  vernier  may  equally  well  be 
fixed  in  such  manner,  that  its  lower  extremity  vt  (or  10  of  the  vernier) 
transfers  the  top  of  the  mercury  to  the  scale,  as  the  same  number  at  the 
coincidence  7  will  indicate  the  distance  of  its  lower  extremity  from  29.0 
inch  on  the  main  scale,  and  this  point  therefore  be  29.07  inches ;  only  that 
in  this  case  in  looking  from  the  last  counted  division  on  the  scale  along 
the  vernier,  its  numbers  appear  reversed  in  order,  beginning  with  the 
highest. 

74.  When  the  vernier  (so  named  after  its  inventor)  is  so  constructed, 
that  its  ten  divisions  are  equal  to  nine  of  the  main  scale,  it  is  often  called  a 
Nonius,  meaning  the  ninth.     Each  division  of  this  vernier  is  y^th  shorter 
than  the  smallest  division  of  the  main  scale.     The  principle  and  the  mode 
of  reading  it  off  are  exactly  the  same  as  those  of  the  last  described  vernier, 
(fig.  27),  only  that  when  both  are  fixed  in  the  same  manner,  their  numbers 
always  run  in  the  reversed  order  of  each  other.     Two  similar  verniers 
constructed  by  making  their  20  divisions  equal  to  19  of  the  main  scale, 
are  seen  in  fig.  33,  which  represents  the  upper  portion  of  the  Levelling 
Barometer,  fig.  32,  and  will  be  further  explained  under  it  (81). 

75.  Effect  of  Capillarity.    A  source  of  inaccuracy  in  obtaining  the  true 
height  of  the  mercurial  column  is  caused  by  the  capillary  action  of  the  tube 
on  the  mercury,  by  which  the  latter  is  prevented  from  rising  to  its  proper 
height,  and  thus  instead  of  a  higher  and  level  surface,  presents  one  that  is 
lower  and  convex.     This  error  is,  however,  constant  for  the  same  baro- 
meter, and  may  be  avoided  altogether  by  fixing  the  scale,  as  is  usual  with 
all  excepting  standard  barometers,  not  by  the  actual  height  of  the  mercu- 
rial column,  but  by  comparison  with  a  standard.     Should  this  not  have 
been  done,  this  error  may  be  estimated  from  the  diamater  of  the  bore  of 
the  tube,  as  given  by  the  following  table. 

Table  of  Corrections  for  Capillarity. 


Diameter  of 
Tube. 

Correction  for 
Capillarity. 

Diameter  of 
Tube. 

Correction  for 
Capillarity. 

Mercury 
boiled. 

Mercury  not 
boiled. 

Mercury 
boiled. 

Mercury  not 
boiled. 

Inch. 

Inch. 

Inch. 

Inch. 

Inch. 

Inch. 

0.60 
0.50 
0.45 
0.40 
0.35 

0.002 
0.003 
0.005 
0.007 
0.010 

0.004 
0.007 
0.010 
0.014 
0.020 

0.30 
0.25 
0.20 
0.15 
0.10 

0.014 
0.020 
0.029 
0.044 
0.070 

0.028 
0.040 
0.060 
0.088 
0.142 

This  correction  is  always  to  be  added  to'the  observed  height.     The  first 
column  is  to  be  used,  where  all  the  air  and  vapor  adhering  to  the  tube  (54) 

47 


48  BOYE'S  INANIMATE  MATTER. 

have  been  expelled  by  the  boiling  of  the  mercury  (62  and  63) ;  the  second 
column  where  this  is  not  the  case,  and  is  also  applied  to  the  level  in  the 
open  part  of  syphon  barometers. 

76.  Effect  of  Friction  and  Adhesion.    Another  source  of  error,  more  diffi- 
cult to  guard  against,  is  caused  by  the  friction  of  the  mercury  against,  and 
its  adhesion  to  the  tube,  by  which,  instead  of  moving  freely  up  and  down 
by  any  change  in  the  pressure  of  the  atmosphere,  it  remains  attached  to  the 
sides,  so  that  when  it  is  falling,  it  at  first  exhibits  a  less  convex  and  after- 
wards even  a  concave  surface,  and  may  at  last  be  prevented  from  falling 
any  further  by  the  mercury  on  the  sides  not  following  it ;  when,  on  the 
contrary,  it  is  rising,  it  exhibits  a  more  convex  surface  than  it  ought,  and  is 
also  prevented  by  the  adhesion  of  the  mercury  to  the  sides,  from  rising  to 
its  proper  height.     To  avoid  this  error  the  mercury  should  always,  before 
taking  an  observation,  be  put  in  motion  by  gently  tapping  with  the  finger  on 
the  outside  of  the  tube;  or,  if  the  barometer  be  suspended  so  as  to  swing 
freely,  a  moderate  motion  may  be  imparted  to  the  whole  instrument. 

77.  Effects  of  Temperature.    As  mercury  expands  by  heat  and  thus 
diminishes  its  density  or  specific  gravity,  it  will  require  a  proportionally 
greater  height  to  counterbalance  the  same  pressure,  and  it  therefore  becomes 
necessary  to  refer  all  observations  to  a  standard  temperature.     This  stand- 
ard temperature  is  generally  assumed  at  32°  Fahrenheit,  or  0°  Centi- 
grade.    The  temperature  of  the  mercury  must,  therefore,  be  ascertained, 
at  the  same  time  that  its  height  is  observed,  by  a   separate  thermome- 
ter, with  which  all  accurate  barometers  are  furnished,  see  I  fig.  28  and 
g  fig.  32 ;  and  if  the  mercury  be  not  of  this  standard  temperature,  the 
observed  height  must  be  reduced  to  the  true  height,  that  is,  the  height 
it  would   have,   if   it    had    the    standard   temperature,   by   applying    a 
correction   to   it.      As   mercury   expands   for   every   degree   Fahrenheit 
0.0001001  of  its  volume,  we  obtain  this  correction  by  multiplying  this 
fraction,  first^  by  the  number  of  degrees  above  or  below  32°,  and  then  by 
the  observed  height,  which  correction  is  deducted  if  the  temperature  be 
above  32°,  and  added  if  the  temperature  be  beloio  32°,  or  calling  the 
observed  temperature  t}  and  the  observed  height  h±  the  correction  for  tem- 
peratures above  32°  will  be  =  —  0.0001001  (t— 32°)  h±,  and  for  tempe- 
ratures below  32°  =  -f-  0.0001001  (32°— f)  \.     But  the  scale  also  ex- 
pands by  heat  and  contracts  by  cold  and  is  therefore  only  correct  at  a  cer- 
tain normal  or  standard  temperature,  which  for  English  measures  is  62°. 
Above  this  temperature  we  therefore  measure  the  height  by  too  long  a 
scale  and  obtain  the  height  too  small,  and  we  must  therefore  add  to  the 
observed  height  the  expansion  of  the  scale.     Below  this  temperature  we 

48 


PNEUMATICS.  49 

measure  it  with  too  short  a  scale  and  obtain  the  height  too  large,  and  we 
must  therefore  deduct  from  the  observed  height  this  contraction.  The 
expansion  of  brass  being  for  every  degree  Fahrenheit  0.0000104,  we  obtain 
this  correction  for  a  scale  entirely  of  brass,  and  extending  the  whole  length 
from  the  lower  to  the  upper  level  of  the  barometer,  by  multiplying  this 
expansion,  for  one  degree,  first,  by  the  number  of  degrees  above  or  below 
62°,  and  then  by  the  observed  height,  which  correction  is  to  be  added,  if 
the  temperature  be  above  62°,  and  deducted  if  below,  being  therefore  for 
temperatures  above  62°  ==  -f  0.0000104  (t— 62°)^,  and  for  temperatures 
below  62°  =  —  0.0000104  (62°— *.)&t.  These  two  corrections  for  the  ex- 
pansion of  the  mercury  and  the  scale  will  be  found  to  counteract  each 
other  at  29°,  which  is  therefore  the  only  temperature  at  which  no  correc- 
tion  is  necessary.  :'5 

If  the  barometer  be  French,  and  therefore  have  a  scale  of  millimeters 
and  a  Centigrade  thermometer,  we  have,  that  the  expansion  of  the  mercury 
for  each  degree  Centigrade  is  0.0001802,  and  for  brass  0.0000188.  But 
as  the  standard  temperature  for  French  measures  is  0°  Centigrade,  the 
same  as  for  the  mercury,  we  may  deduct  the  expansion  of  the  brass  from 
that  of  the  mercury,  leaving  only  one  correction  for  both,  being  for  each 
degree  Centigrade  0.0001614,  which  fraction  we  multiply,  first,  by  the 
number  of  Centigrade  degrees,  and  then  by  the  observed  height,  being  for 
temperatures  above  0°  =  —  (0.0001614x0  \  and  for  temperatures 
below  0°  ==  +  (0.0001614X0  *r* 

Where  many  such  corrections  are  to  be  made,  they  are  most  conveniently 
performed  by  the  aid  of  a  table,  for  which  purpose  see  Table  II,  at  the  end 
of  Pneumatics  page  .  For  a  more  complete  table  the  reader  is  referred 
to  Meteorological  Tables  prepared  by  Guyot  and  published  by  the  Smith- 
sonian Institution. 

If  the  scale  be  engraved  on  the  glass  tube,  and  therefore  of  glass,  the 
expansion  of  glass  must  be  substituted  in  the  above  formulas  for  that  of 
brass,  being  for  1°  Fahrenheit  0.0000045  and  for  1°  Centigrade  0.0000081. 
Where  the  scale  is  a  brass  plate  fastened  on  wood,  no  accurate  correction 

*  The  above  expansions  ought  properly  to  be  referred  to  the  true  height  (32°)  instead  of 
the  observed  height.  The  accurate  and  complete  formula  for  this  correction  for  tempera- 
ture is — — ^— —  HI ;  in  which  ht  =  the  observed  height;  m  =  cubical  expansion 

1-^-771  (£ 1) 

of  the  mercury  for  1  degree,  I  =  the  linear  expansion  of  the  material  of  the  scale  for  1  de- 
gree; t  =  the  observed  temperature  of  the  mercury;  T  =  the  standard  temperature,  to 
which  the  observed  mercurial  height  is  to  be  reduced;  5  the  normal  or  standard  tempera- 
ture, to  which  the  scale  must  be  reduced  in  order  to  be  correct,  and  which  for  English  mea. 
sures  of  brass  is  at  62°. 

D  49  5 


50 


BOYE'S  INANIMATE   MATTER. 


Fig.  28. 


Fig.  29. 


d 


can  be  made  for  the  temperature.  As  wood  is  also  influenced  by  mois- 
ture, and  its  expansion  by  heat  very  small  (about  one-half  that  of  glass), 
it  is  common  to  apply  in  such  cases  only  the  correction  for  the  expansion 
of  the  mercury,  which  will  be  found  in  Table  I,  at  the  end  of  Pneuma- 
tics. By  deducting  from  these  corrections  4\jth  of  their  amount  for  wood, 
and  ^th  for  glass,  they  will  be  sufficiently  accurate  for  all  purposes. 

We  will  now  describe  a  few  mercurial  barometers  as  intended  for  special 
purposes,  and  point  out  their  peculiarities. 

78.  On  board  vessels  a  barometer 
"T  is  required  of  at  least  moderate  accu- 
racy, but  of  simple  construction,  so  as 
not  to  be  liable  to  get  out  of  order. 
It  is  also  desirable  to  avoid  metallic 
scales  or  cases,  as  they  are  liable  to 
rust  by  the  moist  air  and  salt  water. 
Fig.  29  represents  the  inner  arrange- 
ment of  such  a  Marine  Barometer, 
and  fig.  28  shows  it  fixed  in  its  case 
and  suspended.  The  cistern  is  made 
of  wood,  in  one  piece  with  the  cover, 
into  which  the  tube  is  cemented.  The 
bottom,  being  in  part  of  skin,  see 
c  fig.  29,  is  screwed  on  before  invert- 
ing it.  The  cistern  at  g,  where  the 
mercury  rises  and  falls,  is  widened, 
to  diminish  the  error  arising  from 
a  change  in  its  level  (64.)  The  greater 
portion  of  the  tube,  as  far  as  d,  is 
contracted  to  a  very  small  diameter  to 
avoid  violent  oscillations.  The  tube 
with  the  cistern  is  introduced  into 
the  case  by  unscrewing  the  lower  part 
of  h  fig.  28,  which  is  of  brass.  The 
rest  of  the  case  b  5,  is  of  wood, 
widening  at  the  top  into  a  box  with 
glass  front  and  containing  the  scale. 
The  latter  is  of  ivory  and  is  divided  into  tenths  of  inches,  with  a  vernier 
vfig.  29,  also  of  ivory  and  worked  by  a  rack  and  pinion,  the  head  of  which 
pfig.  28,  is  on  the  outside.  The  inches  and  tenths  of  inches  are  read  off 
on  thelnain  scale,  and  the  number  on  the  vernier  at  the  coincidence  added 
as  hundredths,  as  explained  in  72  by  fig.  27.  It  is  suspended  from  a 

50 


PNEUMATICS. 


51 


bracket  i  by  a  universal  joint  &,  so  as  to  remain  perpendicular  during  the 
motions  of  the  ship. 

79.  For  accurate  barometric  observations  through  long  and  rough  jour- 
neys over  mountainous  regions,  particularly  with  a  view  to  estimate  the 
relative  elevations  of  the  country,  GayLussac's  Portable  Syphon  Barometer 
is  generally  employed.     It  consists  of  a  plain  syphon  barometer,  enclosed 
in  a  brass  tube  with  longitudinal  openings  for  viewing  the  mercury,  on  the 
edges  of  which  the  scales  are  engraved,  each 
limb  being  furnished  with  a  separate  scale,  see 
s  and  s  fig.  30,  to  measure  the  distance  from  Fig.  30.     Fig.  31. 

an  arbitrary  horizontal  line  a  between  them, 
which  two  measures  added  together,  give  the 
height  of  the  mercurial  column,  as  explained 
in  65.  As  this  and  other  barometers  are  fre- 
quently imported  from  France,  its  scales  are 
generally  in  French  Millimeters.*  Each  scale 
has  its  separate  sight,  consisting  of  a  ring 
moving  on  the  outside  of  the  brass  tube,  with  a 
vernier  attached  giving  tenths  of  millimetres. 
The  glass  tube,  as  represented  in  fig.  30,  has  the 
lower  extremity  of  the  long  limb  and  the  bend 
c  contracted  to  a  capillary  diameter,  so  that 
any  violent  motion  of  the  mercury  is  checked, 
and  by  inverting  it,  in  which  position  trans- 
portable barometers  are  always  carried,  the 
mercury  remains  in  the  bend  c  as  represented 
in  fig.  31,  and  to  prevent  the  mercury  in  the 
rest  of  the  short  limb  from  escaping  from  it, 
this  is  closed  at  its  extremity,  which  receives 
the  mercury,  but  a  small  capillary  orifice  n, 
with  the  edges  turned  inward,  is  left  at  a  short 

distance  from  it  to  admit  the  am  Near  the  extremities  of  both  limbs 
at  I  and  I,  the  tube  is  contracted  to  prevent  the  mercury  from  stri- 
king forcibly  against  the  ends  by  inversion  or  by  violent  jolting, 
by  which  barometers  are  liable  to  be  broken.  To  prevent  the  same 
when  carrying  ordinary  barometers  which  cannot  be  inverted,  they 


*  For  the  conversion  of  millimeters  into  English  inches,  and  of  English  inches  into  milli- 
meters, a  table  will  be  found  at  the  end  of  Pneumatics. 


51 


52 


BOYE'S  INANIMATE  MATTER. 
Fig.  32. 


should  first  be  inclined  gently  till  the  mercury  fills  the  whole   empty 
space  at  the  top  of  the  closed  limb,  and  then  be  carried  in  this  poskion. 

62 


PNEUMATICS.  53 

80.  As  by  rough  travelling,  in  spite  of  all  precautions,  a  bubble  of  air 
will  sometimes  get  into  the  tube,  to  prevent  it  in  such  cases  from  getting 
into  the  vacuum  at  the  closed  end  and  thereby  spoiling  the  instrument, 
when  its  use  may  be  important,  and  when  it  could  not  be  replaced,  the 
above  and  other  similar  barometers  have  sometimes  the  tube  drawn  out 
at  some  part  into  a  capillary  extremity,  see  d  fig.  80,  and  this  set  and  sealed 
into   another  tube,  which   thus  forms    the    continuation  of  the  former. 
Any  small  air-bubble,  that  should  find  its  way  into  the  tube,  will  then 
be  intercepted  at  this  place,  between  the  capillary  termination  and  the  tube 
surrounding  it,  where  it  may  remain,  doing  no  harm,  until  it  can  be  re- 
moved by  inversion  or  other  suitable  or  necessary  treatment.     It,  however, 
increases  the  liability  of  the  tube  to  break  at  this  place. 

81.  Where  portability  is  required  in  connection  with  the  greatest  scien- 
«tific  accuracy,  as  for  very  accurate  levelling  purposes,  the  form  represented 

in  jig.  32,  is  used.  This  Levelling  Barometer  is  a  cistern  barometer. 
The  sides  of  the  cistern  are  formed  of  a  short  glass  cylinder  c,  the  edges 
of  which  are  ground  true,  the  upper  being  pressed  firmly  against  a  wooden 
(box-wood)  top,  through  the  centre  of  which  the  barometer-tube  passes 
loosely,  being  secured  to  it  by  an  annular  piece  of  kid,  the  inner  edge  of 
which  is  tied  around  the  tube,  its  outer  edge  round  the  projecting  edge 
of  the  opening  in  the  cover.  The  lower  edge  of  the  glass  cistern  is 
pressed  tightly  against  a  wooden  ring  furnished  below  with  an  external 
screw,  on  which  may  be  screwed  another  wooden  ring,  around  which 
a  piece  of  kid  is  secured  bag-fashion,  forming  the  bottom  of  the  cis- 
tern for  containing  the  mercury.  The  above  wooden  top  and  bottom 
pieces  are  secured  against  the  glass  cistern  by  two  annular  brass  pieces 
forced  together  by  three  bolts  as  seen  at  c.  The  lower  brass  piece  is  fur- 
nished with  a  screw,  on  which  may  be  screwed  the  brass  cylindrical  case  A, 
having  through  its  bottom  a  screw,  by  which  the  mercury  in  the  cistern 
may  be  raised  or  lowered  to  the  beginning  of  the  scale,  indicated  by  its 
just  touching  an  ivory  point,  seen  inside  the  cistern  near  p,  projecting  down- 
wards from  the  top  of  the  cistern  as  described  in  64.  To  the  upper  brass 
piece  of  the  cistern,  the  long  upright  brass  tube  Jc  is  fixed,  enclosing  the 
barometer  tube,  see  1 1  fig.  33,  containing  the  mercurial  column.  To 
observe  this  the  brass  tube,  see  If- fig.  32,  and  k  Is  fig.  33,  is  furnished  from 
its  middle  upwards  with  two  longitudinal  openings  on  its  opposite  sides, 
along  the  edges  of  which  the  scale  is  engraved,  which  on  the  one  side  may 
be  French  Centimetres,  on  the  other  English  Inches.  The  arrangement 
of  the  sight  and  vernier  or  nonius  is  seen  of  natural  size  in  fig.  33. 
The  sight  consists  of  a  ring,  the  two  lower  edges  of  which  z  and  the  cor- 
responding one  on  the  other  side,  form  the  sight-line,  which  is  to  be 

63 


BOYE'S   INANIMATE  MATTER. 


33. 


brought  on  a  level  with,  the  top  of  the  mercurial  column.  For  this  pur- 
pose this  ring  is  attached  by  the  screw  r  to  the  piece  m  n,  of  which  n  slides 
on  the  outside  of  the  brass  tube  k  k,  and  is  moved  up  and  down  by  the 
hand,  until  the  sight  z  is  near  the  top  of  the  mercury.  We  then  turn 

the  piece  m,  which  is  movable  round  the  piece 
n,  with  which  it  is  connected,  and  by 
the  fine  screw  r  works  on  the  sight  z,  so  that 
this  latter  is  moved  still  further  towards  the 
top  of  the  mercury,  till  at  last  the  light  seen 
between  them  through  the  tube  just  disappears 
in  the  middle,  by  their  apparently  touching 
each  other,  when  the  sight-line  z  is  exactly 
on  a  level  with  the  top  of  the  mercury.  The 
highest  division,  or  20,  of  both  verniers  v  and 
v  then  indicates  the  point  on  their  respective 
scales,  which  corresponds  to  the  top  of  the 
mercury.  To  read  this  off  on  the  right  hand 
side,  the  numbers  on  the  scale  are  French 
centimetres,  divided  into  tenths.  As  the  ver- 
nier has  20  divisions,  it  subdivides  the  tenths 
of  centimetres  again  into  twentieths  and  thus 
(10X20=200)  gives  two-hundred  ths  of  cen- 
timetres. We  therefore  obtain  the  height 
of  the  mercurial  column  by  reading  off  on  the 
main  scale  the  centimetres  and  tenths  to  the 
nearest  lower  division,  which  is  77.4,  and 
adding  to  this  the  distance  from  it  to  the 
extremity  of  the  vernier,  by  taking  the  number 
of  that  division  of  the  vernier,  which  coincides 
with  a  division  on  the  scale,  being  3  two-hun- 
dredths  (3^),  which  by  dividing  by  2  are 
converted  into  one-hundredths  =  1.5  hun- 
dredths=:0.015,  which  added  to  the  above 
77.4  gives  77.415  centimetres,  or  774.15 
millimetres,  as  the  whole  height. 

To  read  off  the  left  hand  side,  it  will  be  seen  that  the  scale  is  divided 
into  inches,  tenths,  and  half  of  tenths,  that  is  twentieths  (^  =  0.05).  As 
the  vernier  has  20  divisions,  it  subdivides  the  twentieths  of  inches  into  20 
parts  and  thus  gives  (20x20=400)  four-hundredths  of  an  inch.  On  the 
main  scale  we  therefore  read  off  the  nearest  lower  division,  which  is  30.45, 

54 


VD 


PNEUMATICS.  55 

U 

to  which  we  add  the  distance  to  the  extremity  of  the  vernier,  by  taking  the 

number  of  that  division  of  the  vernier,  which  coincides  with  a  division  of 
the  scale,  being  13  four-hundredths  (=  -^Q),  which  divided  by  4 
to  convert  them  into  hundredths  gives  3.25  hundredths  =  0.0325,  which 
added  to  30.45  gives  30.4825  inches,  as  the  whole  height  of  the  mercurial 
column.  It  would  have  been  more  convenient,  if  this  vernier  had  contained 
25  divisions,  so  as  to  subdivide  the  twentieths  of  the  scale  into  25  parts 
instead  of  20,  as  then  the  divisions  of  the  vernier  would  have  indicated 
500ths  so  that  multiplied  by  2  they  would  be  converted  into  lOOOths  of  an 
inch. 

The  enclosing  brass  tube  of  this  barometer,  Jc  Jig.  32,  contains  the 
bulb  of  a  thermometer  #,  to  ascertain  the  temperature  of  the  mercury 
in  the  tube,  and  which  should  be  read  off  immediately  after  adjusting 
the  sight  z,  as  the  proximity  of  the  observer  is  apt  to  increase  the  tem- 
perature, while  reading  off  the  vernier.  This  brass  tube  is  also  fur- 
nished with  two  small  transverse  axes,  of  which  one  is  at  its  middle, 
by  which  it  is  suspended  when  in  use  by  a  universal  motion  in  the  top 
w  of  a  three-legged  mahogany  stand,  q  q  q.  When  required  to  be  fixed 
for  transportation  the  barometer  is  lowered,  so  as  to  be  suspended  in 
the  stand  at  w  by  the  other  or  upper  axis  a.  The  screw  at  h  is  then  turned 
till  the  mercury  is  nearly  up  to  the  top  of  the  cistern,  the  stay-wires  i  i  i, 
are  raised,  and  the  legs  of  the  stand  which  move  on  hinges  are  folded 
together,  so  as  to  form  an  enclosing  case  around  the  barometer.  The 
whole  is  then  cautiously  turned  upside  down,  and  having  secured  the  legs 
of  the  stand  together  by  a  brass  ring,  slipped  into  a  leather  case. 

82.  Standard  barometers  have  a  similar  construction  to  the  Levelling 
barometer,  Jig.  32,  only  as  they  remain  permanently  fixed  in  one  place, 
they  are  generally  made  of  a  much  larger  diameter,  so  as  to  avoid  altogether 
the  capillary  action  of  the  tube  on  the  mercury.     Instead  of  the  ordinary 
sight,  they  are  sometimes  furnished  with  a  sliding  telescope,  moved  by  a 
rack  and  pinion.     The  vernier  which  is  attached  to  it,  is  read  off  by  the  aid 
of  a  magnifier  and  is  so  constructed  as  to  indicate  thousandths  of  an  inch. 

83.  For  stationary  observatories  the  mercurial  barometer  may  be  made 
self-registering,  the  mode    and  principle  of  which  will  be  described  in 
another  place  (see  Thermics  under  Thermometers). 

84.  Although  the  changes  in  the  mercurial  barometer  are  small,  still 
with  proper  precautions,  they  are  always  given  in  the  right  proportion. 
The  mercurial  barometer  with  the  previously  described  improved  means 
of  observing  and  measuring,  constitutes  therefore  the  most  accurate  instru- 
ment which  we  have  for  measuring  the  pressure  of  the  atmosphere,  the 
only  objections  to  it  being  its  weight  and  liability  to  break  by  transportation. 

55 


56 


BOYE'S   INANIMATE   MATTER. 


But  even  in  this  latter  point  it  has  the  advantage,  that  it  rarely  deceives. 
To  obviate,  however,  these  objections,  several  substitutes  have  been  in- 
vented, which  we  will  now  describe. 

Substitutes  for  the  Mercurial  Barometer. 

85.  In  the  mercurial  barometer  we  measure  the  atmospheric  pressure 
by  the  weight  of  the  mercury,  due  to  the  action  of  Gravity.     Instead  of 
gravity  may  be  substituted  the  Elasticity  of  bodies,  such  as  that  of  perma- 
nent gases,  of  vapors,  or  of  metals. 
Fig.  34. 


Mies- 


86.  In  Adie's  Sympiesometer  (from  ffu/ixteZw  (sym- 
piezo),  I  compress,  and  ^erpov  (metron)  measure),  the  elas- 
ticity of  a  confined  gas  is  used  to  estimate  the  pressure  of  the 
atmosphere.  It  is  made  of  glass,  see  fig.  84,  and  has  the 
general  appearance  of  a  syphon  barometer,  but  is  much 
shorter,  and  the  closed  end  a  considerably  enlarged  to 


contain   the 


gas, 


which  should   be  one  that  does   not 


act  on  oil,  such  as  Hydrogen  or  Nitrogen,  generally  the 
former.  The  bend  d  with  part  of  the  open  end  c  is  filled 
with  oil,  which  thus  confines  the  gas  by  separating  it 
from  the  atmosphere.  It  will  thus  be  seen,  that  when 
the  pressure  of  the  atmosphere  on  the  oil  in  the  open 
end  at  c  varies,  the  volume  of  the  gas  in  a  is  diminished 
or  increased  according  to  Mariotte's  law,  and  the  level 
of  the  oil  at  b  thereby  altered.  But  the  volume  of 
the  gas  is  also  altered  by  the  temperature.  To  correct 
it  for  this  influence,  without  the  aid  of  calculation,  the 
scale  e,  which  is  to  indicate  from  the  volume  of  the  gas 
the  pressure  of  the  atmosphere  in  inches,  corresponding  to  the  mercurial 
barometer,  is  made  movable,  its  index  i  sliding  over  another  but  fixed 
scale  s,  wnich  contains  numbers  corresponding  to  the  different  tem- 
peratures of  the  gas,  as  indicated  by  a  delicate  thermometer  I,  which 
is  inverted  in  order  to  have  its  bulb  near  the  gas.'  Before  reading 
off  the  pressure,  the  temperature  of  the  gas  is  observed  with  great 
accuracy  by  the  thermometer  I,  and  the  index  i  of  the  pressure  scale  e 
is  next  placed  on  the  exact  number  of  the  fixed  scale  s,  which  corresponds 
to  the  temperature,  and  the  pressure  is  then  read  off,  as  indicated  by  the 
level  of  the  oil  at  6,  without  any  further  correction.  The  Sympiesome- 
ter is  said  to  indicate  the  changes  in  the  pressure  of  the  atmosphere  much 
sooner  than  the  mercurial  barometer,  and  is  therefore  mainly  used  on  board 
vessels,  for  prognosticating  the  sudden  and  dangerous  squalls  experienced 

56 


PNEUMATICS. 


57 


in  the  tropical  regions.  To  prevent  the  effect  of  the  motion  of  the  vessel, 
its  diameter  is  often  contracted  near  the  bend  at  d.  The  Sympiesometer 
in  its  present  form  is  due  to  Mr.  Adie,  of  Glasgow,  and  only  those  made 
by  him  and  bearing  his  name  are  considered  as  good  and  reliable.  It  has, 
however,  the  inconvenience,  that  the  oil  is  apt  to  become  thick  by  the  action 
of  the  air  at  the  open  end,  by  which  the  working  of  it  is  impaired.  To 
prevent  the  oil  from  spilling,  by  transportation,  from  the  open  end,  this  is 
generally  furnished  with  a  stopper,  which  can  be  drawn  down  on  it  by  a 
wire,  projecting  from  the  lower  edge  of  the  instrument  and  furnished  with 
a  nut  to  tighten  it. 

87.  Another  substitute,  is  the  Boiling  Point  Barometer,  generally  known 
under  the  name  of  the  Boiling  Point  or  Hypsometric  Thermometer,  which 
acts  on  the  principle  of  estimating  the  atmospheric  pressure  from  the  elas- 
ticity or  tension  of  the  vapor,  generated  from  pure  water,  boiling  in  an 
open  vessel,  which  tension  is  the  same  as  the  atmospheric  pressure.  Fig. 


Fig.  35. 


35  represents  the  most  approved  form,  that  of  Regnault, 
(Ann  de  Chimie,  3d  ser.  Vol.  XIV.,  p.  202.)  The  in- 
strument is  made  of  sheet  brass,  and  consists  of  a  small 
cylindrical  vessel  I  k  i,  into  the  lower  closed  end  of 
which  pure  distilled  water  w  is  introduced.  This  part  is 
fixed  into  the  top  of  another  cylindrical  vessel  g  h  m  n, 
the  bottom  of  which  is  formed  of  a  small  spirit  lamp  a  6, 
which  fits  into  it  by  a  catch,  and  by  which  the  water  is 
made  to  boil.  The  necessary  draft  of  air  enters  through  the 
lower  orifices  o  o,  and  passes  out  at  the  upper  ones  o±  o±. 
m  n  is  a  sliding  ring  extending  down  on  one  side,  by 
which,  in  case  of  windy  weather,  the  lower  openings  o  o 
may  be  diminished  or  closed  on  the  side  towards  the 
wind.  The  upper  part  r  t  s  of  the  vessel  Iki  containing 
the  water,  is  formed  of  short  sections  of  tubes,  sliding 
inside  of  each  other,  as  those  of  a  telescope.  The  upper- 
most has  an  opening  o±i  large  enough  for  the  free  escape 
of  the  steam,  and  its  top  is  closed  by  a  stopper  v,  through  q  , 

which  the  stem  of  the  thermometer  u  u  slides  easily,  but 
safely.  The  scale  of  this  thermometer  is  marked  on  the  glass,  and  includes  only 
about  25°  next  below  the  ordinary  boiling  point,  each  degree  being  very  large, 
and  divided  into  tenths  and  even  smaller  fractions.  Having  introduced  2  or 
3  cubic  inches  of  distilled  water,  the  alcohol  lamp  is  lighted,  so  as  to  cause 
the  water  to  boil,  while  the  thermometer  is  constantly  adjusted  by 
moving  the  stem  down  through  the  stopper,  so  that  the  top  of  the  niercu- 

57 


58 


BOYE'S   INANIMATE   MATTER. 


rial  column  is  barely  visible  above  it  at  v,  while  by  sliding  the  tubes  s  t  r 
up  and  down,  inside  each  other,  the  bulb  is  kept  at  the  distance  of  about 
one  inch  above  the  water.  When  the  mercury  becomes  stationary,  while 
the  water  is  all  the  time  boiling,  the  exact  temperature  is  read  off  to  the 
smallest  possible  fraction  of  a  degree.  The  elasticity  or  tension  of  the 
vapor  corresponding  to  this  temperature,  is  then  ascertained  from  Table 
VII  at  the  end  of  Pneumatics.  A  well-constructed  instrument  of  this 
kind,  will,  with  all  due  precaution,  give  results  not  varying  from  those  of 
the  Barometer  more  than  from  T§Q  to  ^  of  an  inch,  the  main  source  of 
inaccuracy  being  the  difficulty  of  graduating  a  thermometer  correctly  into 
so  small  parts  of  a  degree,  and  the  liability  of  these  to  alter  by  the  subse- 
quent irregular  contraction  of  the  glass  of  the  bulb  and  even  of  the  stem. 
This  instrument,  being  only  14  inches  in  length  when  drawn  out,  is  more 
portable  and  much  easier  packed  without  danger  of  derangement  or  break- 
age, and  is  therefore  often  used  as  a  substitute  for  the  barometer  on  rough 
travels,  to  estimate  the  height  of  the  different  elevations,  see  94,  &c. ;  but 
for  accurate  levellings  of  small  heights  it  is  not  suitable.  It  will  be  seen 
from  Table  VII,  that,  at  30  inches,  barometric  pressure,  a  diminution  of  y1^ 
degree  in  the  boiling  point  corresponds  to  a  difference  in  the  atmospheric 
pressure  of  about  0.059  inch,  and  will  therefore,  when  the  temperature  of 
atmosphere  is  32°,  indicate  a  difference  in  height  of  51.3  feet. 
-  88.  In  the  Aneroid  Barometer  (the  name  said  to  be  formed  by  the  invert 
tor  from  a  privative  and  pea*  (reo),  I  flow,  intended  to  mean,  without  fluid) 
the  atmospheric  pressure  is  measured  by  the  elasticity  of  a  metallic  spring. 
Its  general  form  and  size,  see  fig.  36,  is  that  of  an  ordinary  chronometer. 
Fig.  36.  Fig.  37. 


Its  interior  is  shown 


.  37.     The  main  part  of  it  is  a  metallic  vacuum 
58 


PNEUMATICS.  59 

vessel  b  d,  having  the  form  of  a  very  short  cylindrical  box  of  about  two  and 
a  half  inches  diameter,  from  which  the  air  has  been  almost,  though  not 
entirely,  exhausted  through  the  opening  at  /,  which  opening,  after  effecting 
the  exhaustion,  is  soldered  up.  The  two  ends  (top  and  bottom)  of  this 
box  are  made  of  thin  corrugated  sheet  copper  or  brass,  strengthened  at 
their  middle,  one  being  fastened  to  the  supporting  plate  of  the  instrument, 
while  the  other  d,  by  the  upright  rod  a,  is  connected  at  w  with  a  one-armed 
lever  in  n  v,  resting  by  its  two  fulcra  at  n  and  n,  on  the  ends  of  two 
uprights  B  B.  It  will  thus  be  seen,  that  the  atmospheric  pressure  on  the 
two  ends  of  the  vacuum  box  d,  will  have  a  tendency  to  force  them  together, 
and  thereby  to  depress  the  end  v  of  the  lever.  To  prevent  this  and 
to  counteract  the  pressure  of  the  atmosphere  on  the  vacuum  box,  this  end 
of  the  lever  is  supported  by  a  spiral  spring  s,  the  other  end  of  which  is 
fastened  to  a  small  plate,  which  rests  on  the  supporting  plate  of  the  instru- 
ment, but  can  be  raised  or  lowered  by  the  screw  A.  It  must  be  evident, 
that  as  the  pressure  of  the  atmosphere  on  the  vacuum-box  increases,  it  will 
compress  the  spring  s,  and  depress  the  end  v ;  when,  on  the  contrary,  it 
decreases,  the  elasticity  of  the  spring  will  again  raise  it.  To  show  this 
motion  the  end  v  is  connected  by  the  rod  v  r  with  the  arm  r,  which  again 
is  connected  with  the  axis  u  by  a  curved  spring  and  the  screw  z,  by  which 
screw  the  length  of  its  leverage  on  the  axis  u  may  be  altered.  To  the 
axis  u  is  again  attached  the  other  arm  cc,  the  two  arms  r  and  x,  together 
with  the  axis  u,  thus  forming  an  angular  lever.  From  the  end  of  the  arm 
#,  a  slender  rod  h  terminating  in  a  chain  c  passes  to  and  round  a  cylinder, 
the  axis  of  which  at  one  end  is  connected  with  a  flat  spiral  hair-spring  y,\ 
and  at  the  other  end  passes  through  the  face  of  the  instrument  and  carries 
its  index  or  hand  i,  which,  see  fig.  36,  traverses  a  graduated  circle  on 
the  face,  which  circle  is  divided  into  parts  marked  as  inches  and  cor- 
responding to  the  height  of  the  mercurial  column-  in  an  ordinary  baro- 
meter. Disregarding  the  peculiar  form  and  exact  relative  position  of  the 
different  parts,  they  may  be  represented,  and  their  mode  of  action  better 
understood  by  a  reference  to  fig.  38.  Fi9-  38- 

When  the  atmospheric  pressure  on  the 
vacuum  box  b  increases,  it  forces  the 
top  d  of  the  latter  further  in ;  by  this  the 
rod  a  depresses  the  lever  n  v,  thereby 
compressing  the  spring  s.  The  end  v 
of  the  lever  n  v,  then  acts  by  the  rod  v  r 
on  the  angular  lever  r  u  x,  which 
again  draws  the  rod  h  and  the  chain  c, 

and  thereby  turns  the  cylinder,  and  the  hand  i,  which  latter  thus  moves 

69 


60  BOYE'S   INANIMATE   MATTER. 

over  the  face  from  left  to  right,  whereby  at  the  same  time  the  spring  y  is 
slightly  coiled.  When,  on  the  contrary,  the  atmospheric  pressure  on  the 
vacuum-box  diminishes,  the  spiral  spring  s  raises'  the  lever  n  v,  which, 
through  the  angular  lever  r  u  x  slackens  the  rod  h  and  chain  c,  and  thus 
allows  the  spiral  spring  y  to  turn  the  cylinder  back  again,  whereby  the 
index  is  moved  in  the  opposite  direction,  from  right  to  left. 

To  set  the  hand  to  correspond  with  a  standard  mercurial  barometer,  it  is 
moved  by  turning  the  small  screw  A,  fig.  37,  the  head  of  which  will  be 
found  on  the  back  of  the  instrument.  If  then  the  space,  moved  over  by 
the  changes  of  the  atmospheric  pressure  (its  rate  of  motion),  does  not  cor- 
respond with  the  mercurial  barometer,  it  is  adjusted  inside  by  the  screw  z, 
see  fig.  37,  which  alters  the  length  of  the  arm  r.  The  face  of  this  instru- 
ment is  generally  furnished  with  a  thermometer,  see  fig.  36,  the  bulb  of 
which  is  inside,  and  thus  indicates  the  temperature  of  the  instrument. 
The  inventor  of  the  construction  of  this  barometer,  Mr.  Yidi  of  Paris, 
claims  however  to  have  rendered  it  independent  of  the  influences  of  the 
temperature,  by  leaving  a  certain,  very  small,  portion  of  gas  in  the  vacuum- 
box.  The  preponderating  effect  of  heat  on  this  instrument,  he  asserts  to 
be  to  weaken  the  elasticity  of  the  vacuum-box  and  of  the  spring  s,  and 
thereby  to  increase  the  compressing  effect  of  the  atmosphere  on  the  vacuum- 
box,  and  that  this  effect  therefore  may  be  counteracted  by  leaving  in  it  a 
very  small  portion  of  air,  just  enough  to  counterbalance,  by  its  increased 
elasticity  by  heat,  the  increased  compression  of  the  vacuum-box,  in  conse- 
quence of  the  diminished  elasticity  of  the  spring  s.  To  test  the  instru- 
ment for  the  completeness  of  this  compensation  for  temperature,  it  is  only 
necessary,  while  the  atmospheric  pressure  is  found  by  a  mercurial  barome- 
ter to  be  ^stationary,  to  expose  it  to  two  different  temperatures,  and  ascer- 
tain its  variation,  which  variation,  divided  by  the  number  of  degrees  pro- 
ducing this  change,  will  give  the  correction  for  each  degree. 

The  use  of  the  register-hand  t  is,  as  in  the  Wheel-barometer,  merely  to 
note  the  exact  place  of  the  hand  i  of  the  instrument  at  the  last  observa- 
tion, which  is  done  by  moving  it  by  the  small  knob  in  the  centre  of  the 
glass  covering  the  face,  till  it  is  exactly  over  it.  Inspection  at  any  subse- 
quent time  will  then  easily  tell,  how  much  the  barometer  has  risen  or 
fallen. 

The  complicated  construction  of  this  instrument  must  always  render  it, 
for  very  accurate  scientific  purposes,  inferior  to  the  mercurial  barometer. 
Its  extreme  portability,  being  only  of  the  size  of  an  ordinary  chronometer, 
must  however  prove  it  to  be  a  useful,  and,  for  many  purposes,  even  a  highly 
valuable  instrument.  The  great  objection  to  it  is  its  liability  to  get  out 
of  order  without  any  previous  warning  to  the  observer.  When  used  for 

60 


PNEUMATICS. 


61 


Fig.  39. 


important  observations,  it  should  therefore  constantly  be  compared  with  a 
good  mercurial  barometer. 

89.  The  "  Metallic  Barometer"  (Bourdon's)  acts  on  a  similar  principle 

Fig.  39  represents  its  interior  ar- 
rangement, the  face  having  been 
removed  and  the  hands  replaced. 
It  consists  of  a  flat,  hollow,  metallic 
vacuum  vessel  v  v,  made  of  very 
thin  sheet  brass,  of  a  doubly-arched 
or  lenticular  cross-section,  as  seen 
at  the  end  e,  and  curved  as  part  of 
a  hoop,  so  that  the  two  ends  e  and 
e±  are  only  a  short  distance  from 
each  other.  The  outer  side  of  the 
vacuum-vessel  having  a  greater  ex- 
tent of  surface  than  the  inner,  on 
account  of  the  longer  radius  of  its 
curvature,  the  atmospheric  pres- 
sure on  it,  which  acts  at  every  point 
in  the  direction  of  the  radius  of  its 
curvature,  so  as  to  force  it  inward, 
will  be  greater  than  the  pressure 
on  its  inner  side,  which  also  acts 
at  every  point  in  the  direction  of 

the  radius  of  its  curvature,  but  so  as  to  force  it  outward.  The  atmospheric 
pressure  has  therefore  a  constant  tendency  to  increase  the  curvature  of  the 
vacuum-vessel,  so  as  to  cause  its  two  ends  e  and  e±  to  approach  each  other, 
which  effect  is  counteracted  by  the  resistance  offered  by  the  elasticity  of  the 
vessel  itself,  so  that  when  the  atmospheric  pressure  increases,  it  will  cause 
the  ends  to  move  still  nearer  to  each  other;  when,  on  the  contrary,  the 
atmospheric  pressure  becomes  less,  the  elasticity  of  the  vessel  will  cause 
them  again  to  recede  from  each  other.  To  increase  this  motion,  the  ends 
e  and  et  are  made  to  act  by  the  rods  r  and  rt  on  the  ends  of  the  two- 
armed  lever  s  s,  to  the  axis  of  which  another  lever  n  n  is  attached,  the 
end  of  which  carries  a  section  of  a  cog-wheel  a  a.  Thia  cog-wheel  acts 
on  a  pinion  i,  the  axis  of  which  passes  through  the  face  of  the  instrument, 
and  carries  the  hand  y,  which  is  thus  made  to  pass  over  a  graduated  circle 
on  the  face,  the  parts  of  which  indicate  the  corresponding  changes  in  the 
mercurial  barometer  in  inches.  When  the  atmospheric  pressure  is  increased, 
.the  hand  y  is  thus  made  to  move  from  left  to  right;  but  when  it  i& 


K 


62  BOYE'S   INANIMATE   MATTER. 

decreased,  the  elasticity  of  the  vacuum-vessel  will  move  it  in  the  opposite 
direction,  from  right  to  left,  to  assist  in  which,  the  small  weight  g  is  attached 
to  a  short  arm,  which  acts  on  the  axis  of  the  lever  s  s,  thus  assisting  in 
forcing  the  ends  e  et  apart,  p  is  the  register-hand,  which  by  being  placed 
over  the  hand  y,  will  indicate,  what  change  has  taken  place  since  the  last 
observation. 

(1  .*s*~~---~~  Nature  of  the  Barometer. 

90.  The  Barometer  measures  the  pressure  of  the  atmosphere.    This  pres- 
sure depends  mainly,  though  not  altogether,  on  the  weight  of  the  atmosphere, 
because  in  many  cases  the  atmosphere  is  not  allowed  to  press  with  its  whole 
weight  on  account  of  the  lateral  or  upward  currents,  which  take  place  in  it 
and  constitute  what  we  call  winds,  while  if  these  currents  should  meet  each 
other  or  have  a  descending  direction,  it  would  increase  the  pressure  beyond 
what  is  due  to  its  weight.     If,  in  a  similar  manner,  the  air  near  the  earth 
should,  from  some  cause,  become  suddenly  heated,  so  as  to  have  its  elas- 
ticity increased,  it  would  require  some  time  to  put  the  surrounding  air  in 
motion ;  this  would  meanwhile  increase  the  pressure  beyond  what  is  due 
to  its  weight  alone.     In  this  latter  case  it  will  also  be  seen,  that  the  specific 
gravity  or  density  of  the  air  would  not  be  increased.     From  these  considera- 
tions, it  will  be  evident,  that  the  barometer  cannot  correctly  be  said  to 
indicate  the  weight  or  the  density  of  the  atmosphere,  but  only  its  pressure. 

91.  The  Manometer.  To  indicate  the  changes  in  the  specific  gravity  or 
density  of  the  atmosphere,  a  separate  instrument  was  proposed  by  Otto  Gue- 
ricke,  called  the  Manometer,  from  ftavos  (inanos),  rare,  and  /Jter^ov  (metron), 
measure,  meaning,  measurer  of  the  density  of  the  air.*   It  consists  of  two  balls 
of  nearly  the  same  weight,  but  of  very  different  diameters,  the  one  being  hol- 
low, the  other  solid,  both  suspended  to  a  balance-beam,  so  as  to  counter- 
poise each  other  in   the  air.     As   bodies  suspended  in   a  fluid   lose   as 
much  of  their  weight,  as  the  volume  of  the  fluid  which  they  displace 
weighs,  it  will  be  seen  that  the  larger  ball,  displacing  a  larger  volume  of 
air,  has,  under  the  above  circumstances,  lost  more  of  its  absolute  weight, 
than  the  smaller;  and  that  any  change  in  the  density  of  the  air  will  affect 
it  more  than   the  smaller,  detracting   more  from  its  weight,  when   the 
density  becomes  greater,  and  restoring  to  it  more  of  the  weight,  already 
lost,  when  the  density  becomes  less.     In  either  case,  therefore,  the  equili- 
brium of  the  balance  will  be  destroyed,  the  large  ball  rising  when  the 

*  The  word  Manometer  is  sometimes,  though  incorrectly,  applied  to  pressure-guages, 
see  (105),  for  measuring  the  tension  or  elasticity  of  gases,  this  latter  being  considered  pro- 
portional to  their  density;  but  in  the  confined  state,  the  elasticity  is  altered  by  the  tempe- 
rature, while  the  density  is  not  affected,  unless  they  be  vapors  in  contact  with  a  liquid. 

62 


PNEUMATICS.  63 

density  of  the  air  is  increased,  and  falling  when  it  is  diminished.  For  the 
same  reason,  very  light  and  bulky  substances,  such  as  feathers,  have  a  per- 
ceptibly greater  weight  than  that  ascertained  in  the  ordinary  way  in  air. 
Hence  as  a  puzzle  it  may  be  said,  that  a  pound  of  feathers  is  heavier  than 
a  pound  of  lead.  /^)  IT 

J^J |_||  Uses  of  the  Barometer.  J  /J 

J{J  92.  As  a  weather-glass.  The  most  popular  use  made  6£  the  barometer  is 
for  prognosticating  the  weather.  The  weather  is  said  to  be  bad  when  either 
windy,  or  rainy,  or  both.  Winds  are  masses  of  air  in  motion.  Their 
direction  being  mostly  lateral  and  upward,  their  most  frequent  effect  on 
the  barometer  is  to  prevent  the  air  from  pressing  with  its  whole  weight  on 
it,  and  thus  in  most  cases  to  cause  the  barometer  to  fall,  but  not  necessarily 
so,  since  if  the  wind  have  a  downward  tendency,  it  will  cause  the  barometer 
to  rise.  Rain  is  most  frequently  caused  by  a  hot  and  moist  current  of  air 
mixing  with  a  cold  current  or  passing  over  a  cold  country,  or  by  the  ascent  of 
a  heated  column  of  air  saturated  with  moisture,  in  which  case  the  moisture  con- 
denses by  the  cold  produced  by  the  expansion  of  the  air,  on  account  of  the 
less  pressure  as  it  ascends.  Rain  thus  depends  more  or  less  on  currents  of 
air,  either  near  the  surface  of  the  earth  or  higher  up,  which,  as  we  have  just 
seen,  will  affect  the  barometer;  and  thus  rain,  like  wind,  generally  causes  the 
barometer  to  fall,  but  not  necessarily  so,  as  it  may  even  have  a  contrary  effect. 
The  condensation  of  the  vapor  and  its  consequent  withdrawal  from  the 
atmosphere,  causes  also  a  certain,  but  comparatively  very  small,  depression 
of  the  barometer.  As  thus  changes  in  the  barometer  nearly  always  accom- 
pany changes  in  the  weather  and  frequently  precede  them  by  more  or  less 
time,  this  instrument  serves  as  an  excellent  guide  to  account  for  present, 
and  even  to  anticipate  coming  changes  in  the  weather. 
*""  The  Wheel  and  other  cheap  barometers  constitute,  therefore,  the  common" 
and  most  popular  Weather-Glass,  and  for  this  purpose  instrument-makers 
have  affixed  to  different  parts  of  the  scale  certain  inscriptions,  indicative  of 
the  weather,  viz.  "  Fair,"  at  the  average  stand  of  30  inches;  "  Change,"  at 
29.5;  "Rain,"  at  29;  "Much  Rain,"  at  28.5,  and  "Stormy,"  at  28; 
while  above  30  inches  we  find  "  Set  Fair,"  at  30.5,  and  "Very  Dry,"  at  31 
inches.  These  inscriptions  are  very  fallacious,  and  have  done  much  harm 
by  bringing  the  barometer  in  disrepute  and  calling  the  attention  away  from 
the  real  scale  of  the  instrument,  which  indicates  the  height  of  the  mercu- 
rial column.  For  although  the  most  severe  gales  are  generally  accom- 
panied by  rain  and  cause  the  lowest  stand  of  the  barometer,  much  rain 
and  bad  weather  may  also  occur  at  a  high  stand,  where  therefore  the 
inscriptions  indicate  fair  weather.  Even  for  such  one-sided  use  of  the 

63  ^ 

1 


64  BOYE'S  INANIMATE  MATTER. 

barometer,  it  would  be  better,  instead  of  having  or  looking  at  the  inscrip- 
tions, to  note  whether  the  barometer  is  in  the  act  of  falling  or  rising,  since 
even  at  a  high  stand  a  considerable  fall  is  most  likely  to  bring  about  a 
change  in  the  weather,  although  the  fall  might  not  reach  the  ominous 
inscriptions  on  the  "  Weather-glass,"  while,  on  the  other  hand,  immediately 
after  a  storm,  fine  weather  often  appears  before  announced  by  the  glass.  To 
find  whether  a  rise  or  fall  has  taken  place  since  the  last  observation,  most 
"  glasses"  are  furnished  with  a  register-hand,  which  is  placed  by  the 
observer  over  the  hand  of  the  instrument,  but  which  register-hand,  in  many 
cases,  is  much  larger  and  more  conspicuous  than  the  index-hand  itself,  so 
as  to  attract  the  sole  notice  of  casual  observers,  and,  by  being  mistaken  for 
the  hand  of  the  instrument,  gives  them  the  idea  of  a  stand  still  in  the  instru- 
ment, when  they  most  expected  a  change.  It  is  given  as  a  rule,  that  a 
change  in  the  weather,  accompanied  by  a  gradual  and  slow  change  in  the 
barometer,  is  likely  to  last  longer  than  those  changes,  which  are  indicated 
by  a  sudden  change  in  the  barometer. 

From  the  nature  of  the  barometer,  as  explained  in  90,  an  intelligent 
observer  will,  therefore,  no  more  expect  bad  weather  to  follow  invariably  a 
fall  or  low  stand  of  the  barometer,  or  good  weather  to  accompany  invari- 
ably a  rise  or  high  stand,  than  he  would  expect  one  kind  of  weather  to 
follow  invariably  a  wind  from  the  east,  and  the  opposite  kind,  one  from  the 
west.  He  will  know  that  changes  may  occur,  which  not  at  all  or  but  slightly 
affect  the  barometer,  and  in  such  cases  he  will  rely  on  other  indications. 
On  the  other  hand,  while  from  a  change  in  the  barometer  he  will  not  expect 
invariably  a  change  in  the  weather,  still  he  will  know,  that  in  such  cases 
causes  are  active,  which  may  bring  about  such  changes.  He  will  consult 
the  Hygrometer  (164),  to  ascertain  whether  the  amount  of  moisture  is  at 
the  same  time  increasing  or  decreasing ;  the  Thermometer,  to  observe  any 
simultaneous  change  in  the  temperature ;  and,  above  all,  by  constant  and 
attentive  observation  and  by  careful  study  of  previous  records,  he  will 
become  familiar  with  the  habits  of  different  winds,  at  the  different  seasons 
of  the  year,  and  in  the  particular  localities.  Thus  he  may  find  that  in  the 
fall  of  the  year,  after  a  long  and  continued  spell  of  dry  and  fine 
weather,  a  change  is  often  anteceded  by  a  rise  in  the  barometer  instead  of 
a  fall ;  that  certain  directions  of  winds  are  more  apt  than  others  to  bring 
about  a  fall  or  rise  without  a  corresponding  change  in  the  weather ;  that 
certain  storms,  which  by  careful  attention  he  will  be  able  to  distinguish, 
are  preceded  by  a  fall,  as  many  of  the  most  violent  and  sudden  tropical 
gales,  others  by  a  rise,  and  others  again  will  at  first  cause  a  fall,  but  then 
a  subsequent  rise  will  indicate  their  increased  violence,  &c.  &c.  In  fact,  to 
unprejudiced  minds,  the  barometer  has  the  advantage  over  all  other  meteoro- 

64 


PNEUMATICS.  65 

logical  instruments,  that  it  indicates  changes  in  the  equilibrium  of  the 
atmosphere,  while  they  are  often  yet  far  distant  from  the  place  of  the 
observer;  and  thus  not  only  puts  him  on  his  guard  against  them,  but  also, 
more  than  any  other  instrument,  guides  him  in  finding  and  studying  their 
causes  and  their  progress. 

93.  As  a  general  rule  the  barometer  is  much  less  variable  in  the  Tropi- 
cal and  Torrid  zones  than  in  the  more  northern  latitudes.  Thus,  in  Peru 
the  range  of  its  variations  is  orily%tf6%t/'i  inch,  while  in  London  it  is  about 
2J  inches,  and  in  St.  Petersburg  over  3  inches.  The  average  stand  of 
the  barometer  at  the  level  of  the  sea  at  45°  latitude  is  30  inches  (or  more 
correctly  29.922  inches),  which  is  considered  as  the  standard  pressure  for 
measuring  gases,  and  to  which,  therefore,  their  volume  is  always  referred 
(100).  It  is  somewhat  higher  near  the  Tropics,  from  which  latitude,  there- 
fore, the  average  stand  of  the  barometer  decreases  both  towards  the  equator 
and  towards  the  poles.  The  average  stand  of  the  barometer  at  any  particular 
place  inland  depends  mainly  on  its  elevation  above  the  level  of  the  sea, 
but  is  also  influenced  to  some  extent  by  the  latitude,  and  by  the  particular 
conformation  of  the  whole  continent,  or  that  part  of  it  where  it  is  situated. 
The  average  stand  of  the  barometer  at  Philadelphia  is  29.95  inches.  The 
barometer  is  subject  to  monthly  variations,  the  greatest  monthly  mean 
pressures  being  those  for  June  and  January ;  the  lowest,  those  for  Novem- 
ber and  March.  At  moderate  latitudes,  the  average  difference  between 
the  means  of  June  and  November  amounts  to  about  0.11  inch.  The 
barometer  also  exhibits  a  regular  diurnal  variation,  standing  highest  at 
nine  o'clock  A.  M.  and  P.  M.,  and  lowest  at  three  o'clock  P.  M.  and  A.  M., 
being  unaffected  by  it  at  noon.  The  hours  of  nine  and  three,  or  at  twelve, 
are  therefore  recommended  as  most  suitable  for  regular  meteorological 
observations  of  the  barometer.  The  average  of  this  daily  variation,  which 
is  ascribed  to  the  heat  of  the  sun,  amounts  in  moderate  latitudes  to  about 
0.03  inch,  but  increases  towards  the  equator  to  0.1'inch;  in  higher  lati- 

Pf~\    tudes  it  is  lost  in  the  irregularcjijinj 

|iifcy  04.-  For  Measuring  lleigMs  or  Levelling  (Hypsometry,  from  u</>o<; 
(hupsos),  height,  and  [usrpov  (metron),  measure).  If  the  atmo- 
sphere were  of  uniform  density  throughout  its  whole  extent,  the  height 
of  the  mercurial  column  in  the  barometer  would  afford  us  an  easy 
means  of  calculating  the  perpendicular  height  of  the  whole  atmo- 
sphere, or  of  any  part  of  it,  from  the  known  laws  of  Hydrostatics,  that 
the  heights  of  columns  of  different  liquids,  equilibrating  each  other 
in  communicating  tubes,  are  inversely  as  their  specific  gravities.  Hence 
it  would  only  be  necessary  to  multiply  80  inches  by  11000,  which  is 
the  number  expressing  how  many  times  mercury  is  heavier  than  air, 
E  65 


66  BOYE'S  INANIMATE   MATTER. 

in  order  to  obtain  the  height  of  the  whole  atmosphere  in  inches,  which 
would  mate  it  about  5.12  miles;  and  in  the  same  manner  the  perpen- 
dicular height  of  any  intermediate  part  of  the  atmosphere,  between  two 
places  not  situated  on  the  same  level,  would  be  obtained  by  multiplying 
the  difference  in  the  stand  of  the  barometer  at  these  two  places  by  the 
same  number,  11000.  But  this  is  not  the  case.  It  has  already  been 
stated  in  27,  that  from  other  experience  it  is  known  that  the  atmosphere 
extends  much  farther;  and  both  reason  and  experience  tell  us,  that  as  we 
ascend  into  the  atmosphere,  the  strata  below  are  not  capable  of  exercising 
any  pressure  on  those  above,  and  that  the  upper  strata,  therefore,  are  sub- 
ject to  less  pressure  and  consequently  have  also  less  density.  In  this 
manner  both  the  pressure  and  the  density  of  the  atmosphere  must  decrease,  as 
we  ascend  from  the  level  of  the  sea  to  greater  elevations :  still,  knowing  the 
exact  ratio  between  the  different  heights  to  which  we  ascend  into  the 
atmosphere,  and  the  decrease  in  the  corresponding  pressures,  the  barome- 
ter will  yet  afford  us  one. of  the  most  valuable  means  to  ascertain  the  differ- 
^_ -i  ences  in  level  of  different  places.  „ 

~~*^$>.  To  understand  the  principle  on  which  this  is  ascertained,  it  may  be 
stated,  that  while  the  different  perpendicular  heights  above  the  surface  of 
the  earth,  if  counted  from  the  upper  sensible  limit  of  the  atmosphere  down 
to  its  lower  limit  at  the-  level  of  the  sea,  form  an  increasing  arithmetical 
progression  (1  ft.,  2  ft.,  3  ft.,  4  ft.,  &c.,  from  the  top  of  the  atmosphere), 
the  corresponding  pressures  on  the  barometer  form  an  increasing  geome- 
trical progression.  Between  any  two  such  series  there  is  a  similar  relation 
as  between  the  ordinary  logarithms  and  their  corresponding  numbers,  the 
logarithms  forming  an  arithmetical  series,  and  therefore  corresponding  to 
the  distances  from  the  top  of  the  atmosphere ;  while  the  numbers,  to  which 
they  are  the  logarithms,  form  a  geometrical  progression,  and  therefore 
correspond  to  the  barometric  pressures.  If,  therefore,  at  the  same  time  or 
moment,  we  ascertain  in  two  different  places,  situated  at  different  heights 
or  on  different  levels,  the  true  barometric  pressures,  that  is,  the  heights  of 
the  mercurial  columns,  corrected  for  the  influence  of  the  temperature  (77), 
and  then  from  an  ordinary  table  of  logarithms  take  the  logarithms  cor- 
responding to  these  two  pressures  (it  matters  not  whether  the  pressures  be 
expressed  in  English  inches  or  in  French  millimeters),  these  two  loga- 
rithms will  indicate  the  relative  distances  of  those  two  places  from  the 
upper  limit  of  the  atmosphere,  and  may,  therefore,  by  multiplying  them  by 
a  constant  number,  be  made  to  give  these  distances  in  English  feet  or  any 
other  measure.  These  distances,  deducted  from  each  other,  will  then,  of 
course,  give  the  difference  in  their  level,  or  the  height  of  the  one  above  the 
other.  To  avoid  the  double  multiplication  of  the  two  logarithms  by  the 

66 


PNEUMATICS.  67 

constant,  the  logarithms  may  first  be  deducted  from  each  other,  and  their  dif- 
ference multiplied  by  it,  which  will  then  give  the  difference  in  their  level. 

To  obtain  these  distances  from  what  may,  with  sufficient  accuracy  for  pre- 
sent purposes,  be  considered  the  upper  sensible  limit  of  the  atmosphere,  in 
English  feet,  the  constant  number  by  which  we  multiply  the  logarithms 
of  the  true  pressures,  is  60158.5,  the  temperature  of  the  atmosphere  being 
supposed  to  be  32°  Fahrenheit,  and  the  difference  between  the  logarithms  of 
the  true  barometric  pressures,  multiplied  by  this  number,  will  therefore  at 
once  give  the  corresponding  difference  in  level  in  English  feet,  the  tempe- 
rature of  the  intermediate  column  of  atmospheric  air  being  32°. 

To  facilitate  these  calculations,  tables  have  been  constructed,  which  give 
the  different  distances  from  the  above  assumed  upper  limit  of  the  sensible 
atmosphere,  calculated  in  this  manner  for  all  the  different  barometric  pres- 
sures. These  distances  for  pressures  from  28  to  31  inches  will  be  found 
in  Table  III  at  the  end  of 'Pneumatics,  page 

But  the  above  distances  are  only  correct  for  the  standard  temperature 
of  the  atmosphere  of  32°.  As  air  expands  by  heat,  and  thus,  with  the  for- 
mation of  an  additional  quantity  of  vapor  of  water,  diminishes  its  density  or 
specific  gravity  for  every  degree  of  Fahrenheit  by  0.00222  of  its  density 
at  32°,  the  same  mercurial  column  will,  at  higher  temperatures,  counter- 
balance a  proportionally  higher  column  of  air.  The  temperature  of  the 
atmosphere  must,  therefore,  always  be  ascertained  at  the  same  time  that  we 
observe  the  pressure,  by  an  accurate  thermometer,  which  has  been  sufficiently 
long  exposed  to  it  in  a  suitable  place.  Should  the  temperatures  at  both 
places  not  be  the  same,  their  average  is  taken  as  the  temperature  of  the 
column  of  air  between  them.  If  then  this  average  temperature  be  not  32°, 
a  correction  must  be  applied  to  the  above  difference  in  level  or  height, 
which  correction  is  obtained  by  multiplying  the  above  given  expansion  of 
the  atmosphere  for  1°  Fahrenheit,  0.00222,  first,  by  the  number  of 
degrees  which  the  average  temperature  of  the  air  is  above  or  below  32°, 
and  then  by  the  above-obtained  height  for  32°,  which  correction  is  to  be 
added,  if  the  average  temperature  of  the  air  be  above  32°,  and  deducted, 
if  below;  or,  calling  the  above  height,  corresponding  to  32°,  hlt,  and 
the  temperatures  of  the  air  at  the  two  places  or  stations,  T  and  T±,  the 

Cor.  for  temp,  of  the  air  above  32°  =  -f  0.00222  (T"{"Ti  —  32°)  hu, 

Cor.  for  temp,  of  the  air  below  32  =  —  0.00222  (  32°  —  T+T*  \  hiv 

It  will  be  seen  that  this  correction  is  quite  considerable.  Thus  at  a 
barometric  stand  of  30  inches,  a  fall  of  ^  inch  corresponds  at  32°  to  a 
difference  in  level  of  87.2  feet,  but  at  80°,  this  correction  for  temperature 

67 


68  BOYE'S   INANIMATE   MATTER. 

of  the  air  being  =  -f  0.00222  X  (80°—  32°)  X  87.2  feet  =  9.3  feet,  it 
will  correspond  to  a  difference  in  level  of  96.5  feet. 

Two  other,  but  comparatively  small,  corrections  are  yet  to  be  applied  to 
the  thus  corrected  height,  on  account  of  the  decrease  of  gravity :  1.  from 
the  poles  toward  the  equator,  2.  from  the  level  of  the  sea  upward  into  the 
atmosphere,  by  which  the  weight  of  the  mercury  becomes  less,  and  the 
same  column  of  mercury  will  thus  counterbalance  a  smaller  column  of  air. 
These  corrections  for  latitudes  near  45°,  and  for  small  heights,  are  often 
for  ordinary  amateur  purposes  entirely  disregarded.  They  will  be  given 
by  Tables  IV  and  V  at  the  end  of  Pneumatics.  The  first  of  them  depends, 
as  stated,  on  the  latitude ;  and  taking  gravity  and  the  consequent  weight 
of  the  mercury  at  45°  latitude  as  standard  or  unit,  we  obtain  this  correc- 
tion by  multiplying  0.0028371,  first  by  the  cosine  of  the  double  latitude,  and 
then  by  the  last-obtained  height,  which  correction,  as  indicated  by  the  sign 
of  the  cosine,  is  to  be  deducted  for  latitudes  greater  than  45°,  and  added 
for  those  less  than  45° ;  or,  calling  the  obtained  height,  corrected  for  tem- 
perature of  air,  \  and  the  latitude  L,  this 

-Co, for  Lat.  =  0.0028371  cos.  2  L.  A,  { ^&ffgEfffr. 

If  the  two  places  have  a  sensible  difference  of  latitude,  the  average  lati- 
tude is  used.  The  second  correction  for  gravity  depends  on  the  height  or 
altitude  itself,  and  is  obtained  by  first  adding  to  the  obtained  height  the 
number  52252,  then  dividing  by  the  mean  radius  of  the  earth,  20886861  ft., 
and  then  again  multiplying  by  the  height,  which  correction  is  always  to 
be  added,  as  on  account  of  the  less  weight  of  the  mercury,  the  upper  por- 
tion of  the  atmosphere  has  given  too  great  a  column  of  mercury  and 
thereby  caused  too  small  a  difference  in  pressure.  Calling  the  height  cor- 
rected for  temperature  of  the  air  and  for  the  latitude,  h0)  the 

Cor.  for  altitude  =  +  ^  +  52252  j, 

20886861  fl°' 

Calling  the  true  height  or  difference  in  level  of  two  places,  7i,  the  barometric 
pressures  at  those  places  corrected  for  temperature  of  the  mercury  and  of  the 
scale,  B  and  b,  the  following  formula  will  give  all  the  different  operations : 

+  0.00222  (2-i-l-i— 32°  ) 


h  =  (Log.  B  —  log.  b)  X  60158,5  ft.  X 


1        or 


—  0.00222   (320  —  -—-^ 


1+0.0028371  cos.  2  L 
h  -f  52252 

,   ""  20886861 


68 


IEUMATICS.  69 

96.  To  illustrate  the  above  by  an  example,  we  may  select  the  calcula- 
tion of  the  height  attained  by  Gay  Lussac  in  his  famous  balloon  ascension 
from  Paris  in  1804,  being  the  greatest  height  ever  attained  in  this  manner. 

Observed  height  of  Barometer  Temp,  of  Merc.         Temp,  of  Air  Lat. 

In  Balloon  =  12.945  inch  =  \    14°.90  =  tt     14°.90  =  T±    IRO*A/  _  T 
At  Paris  =30.145  inch  =  51   87°.44  =  t      87°.44r=T 

Applying  the  corrections  for  temp,  of  the  mercury  and  of  the  scale  (77),  we 
obtain  the 

True  height  of  Barometer 

In  Ball.  =  b,  +  0.0001001  (32°  —  14°.  90)  M         1O  QM  .    ,         , 
1  -  0.0000104  (62°-14°.90)  b\  }  =  12'961  meh  =  b 


At  Par.  =  Bl  —  0.0001001  (87°.44  —  32°)^  \       9Q  Q8q  .    ,         R 
1  +  0.  0000104  (87°.44-  62°;  \B\  }  =  29'983  mch  =  B 

Log.  B  =  Log.  29.983  =  1.4768747 
Log.  b  =  Log.  12.961  =  1.1126365 

(Log.  B—Log.  b}=  0.3642382  =  Difference  of  Logs. 
Difference  of  Logs,  multiplied  by  60158.5 

=  0.3642382  x  60158.5  =  21912.03  feet  =  h 


Average  Temp,  of  Air  =  i±±i  =  51°.17 

Cor.  for  Temp,  of  Air  =  -f  0.00222  /T+yi  _  32' 

=  -f  0.00222  x  19°.  17  x  21912.03  feet  —  +  932.52    " 

Height  of  Balloon,  cor.  for  Temp,  of  Air  =  22844.55  feet  =  hl 

Cor.  for  Lat.  =  0.0028371  cos.  2  L.  hl 

=  —  0.0028371  cos.  97°40'  X  22844.55  feet. 

=  —  0.0028371  X  0.1334097  X  22844.55  feet  =  —  8.65    " 

Height  of  'Ball.  cor.  for  Temp,  of  'Air  and 'for  Lat.  =  22835. 90  feet  =  h0 


22835.90+52252 
=  + 20886861 X  22835.90 /ee<=  +  82.09   « 

Height  of  Balloon  above  Barometer  at  Paris  =  22917.99  feet  =  h 

Add  height  of  Barometer  at  Paris,  above  level  of  sea,  159.78 

Height  of  Ball,  above  the  level  of  the  sea,  =  23077. 77  feet, 

or  4.37  miles. 

By  the  aid  of  Logarithms  these  calculations  are  considerably  facilitated. 

69 


70  BOYE'S   INANIMATE   MATTER. 

97.  To  obtain  the  same  differences  in  level  in  French  metres,  the  constant 
number  for  multiplying  the  difference  of  the  logs,  of  the  two  true  barome- 
tric pressures  (being  in  this  case  generally  obtained  in  millimetres)  is  18336. 
The  expansion  of  the  air  by  heat  and  by  the  addition  of  vapors,  being  for 
every  degree  Centigrade  0.004,  the  correction  for  temperature  is  0.004 

m   i  m  o  srr\   j  rp  \ 

— 2 — -  hu=  — 1QQQ  ^u>  to  be  added  for  temperatures  above,  and  de- 
ducted for  temperatures  below  0°,  as  indicated  by  the  sign  of  (T  -\-  TJ  ; 
the  correction  for  latitude  is  of  course  the  same,  and  that  for  altitude 

is  +  ~6366200  ^°J  ^Q  W^°le  fornmla  ^eing 

2  (T+TJ 
1 T      1000 

h=  (log  B  —  log  b)  X  18336  metres  X  4  1  +  0.0028371  cos.  2  L 

'  /H-15926 

+    6366200 

To  obtain  these  different  heights  in  metres  almost  entirely  by  the  aid  of 

Tables,  the  reader  is  again  referred  to  Meteorol.  Tables  by  Guyot,  published 

by  Smithsonian  Inst.     To  facilitate  the  conversion  of  French  metres  into 

English  feet,  and  of  English  feet  into  French  metres,  Table  VI  will  be  found 

J*L         at  the  end  of  Pneumatics. 

rVV-*U4,  gg  By  calculating  in  the  above  manner  the  height  corresponding  to  a 
y  '  barometric  pressure  of  15  inches,  we  obtain  the  height  of  about  18000  feet 
or  3.4  miles  as  that,  at  which  the  density  of  the  atmosphere  is  only  one- 
half  of  its  density  at  the  level  of  the  sea ;  and  as  the  densities  increase  in 
the  same  geometrical  progression  as  the  pressures,  it  follows  that  if  we 
leave  out  of  consideration  the  effect  of  the  rapid  diminution  of  the  tempe- 
rature of  the  atmosphere  as  we  ascend  higher,  both  the  pressure  and  the 
density  of  the  atmosphere  ought  to  become  one-half  less  for  every  addi- 
tional 3.4  miles. 

99.  For  the  estimation  of  the  difference  in  level  of  two  places  from  the 
barometric  pressures,  only  the  most  accurate  instruments,  such  as  the 
Levelling  Barometer  described  in  81,  fg%.  32  and  33,  should  be  used.  As 
the  barometric  pressure  of  the  atmosphere  is  constantly  changing,  it  is  neces- 
sary to  observe  the  pressures  at  the  same  moment  in  both  places,  for  which 
purpose,  therefore,  two  instruments  are  required,  the  moments  for  observ- 
ing being  indicated  by  signals  or  by  chronometers.  Where  this  cannot 
be  done,  and  the  two  places  are  at  no  very  great  distance  from  each  other, 
the  observer  may  travel  with  his  instrument  from  the  one  place  to  the 
other,  and  then  immediately  back  again  to  the  first  station,  and  if  any 
change  has  occurred,  take  for  this  station  the  average  of  the  two  observa- 

70 


PNEUMATICS.  71 

tions.  If  the  two  places  are  very  distant  from  each  other,  the  average 
stand  of  the  barometer,  derived  from  observations  for  a  length  of  time, 
also  affords  data  from  which  the  difference  in  their  level  is  often  estimated. 
As  the  ordinary  variations  of  the  barometer,  leaving  out  the  extremes, 
which  occur  only  at  considerable  intervals,  rarely  exceed  even  in  moderate 
latitudes  1J  inch,  and  become  much  less  as  we  approach  the  equator  (93), 
observations  with  the  barometer,  performed  on  a  single  journey  over  a 
mountainous  country,  where  therefore  the  differences  in  the  elevations  and 
the  consequent  differences  in  the  barometric  pressures  are  very  great,  will 
afford  data  sufficiently  accurate  for  an  approximate  estimation  of  these 
elevations;  and  the  barometer  is  therefore  the  instrument  commonly 
employed  for  this  purpose,  the  form  combining  the  greatest  portability 
with  sufficient  accuracy  being  that  of  Gay-Lussac's,  described  in  79.  The 
Boiling-Point  Barometer  described  in  87,  though  less  accurate,  has 
been  found  to  give  available  results.  The  Aneroid  and  Metallic  Barome- 
ters, being  the  most  portable  of  all,  have  not  yet  been  sufficiently  tested  for 
such  purposes. 

100.  For  estimating  the  true  volume  of  gaseSj  and  from  it,  their  weight. 
Another  use  of  the  barometer,  for  which  it  is  constantly  required  in  a  che- 
mical laboratory,  is  in  estimating  the  weight  of  a  gas  from  its  volume.  As 
the  volume  of  a  gas  varies  with  the  pressure  on  it,  it  becomes  necessary, 
when  its  volume  is  observed  for  the  purpose  of  estimating  its  quantity  or 
weight,  to  note  the  pressure  by  which  it  is  confined,  and  then  to  reduce 
the  observed  volume  to  what  it  would  be  at  a  standard  pressure,  which  is 
assumed  at  29.9218  inches  of  mercury  (760  milimetres),  this  being  the 
average  stand  of  the  barometer  at  the  level  of  the  sea  at  45°  latitude  (93), 
and  which  number  is  used  for  all  important  estimations,  serving  as  a  basis 
for  other  calculations,  such  as  the  exact  weight  of  100  cub.  inch,  of  air 
(57),  but  for  most  ordinary  purposes  30  inches  is  taken  as  sufficiently" 
accurate.  Suppose,  thus,  that  the  volume  of  a  gas,  confined  in  a  gradu- 
ated glass  tube  by  mercury  or  water  contained  in  a  pneumatic  cistern  (  ), 
be  found  by  the  graduation  of  the  tube  to  be  24  cubic  inches,  when  the 
barometer  stands  at  29  inches,  the  level  of  the  confining  liquid  being  the 
same  inside  the  tube  as  outside.  We  then  have  by  Mariotte's  law  (44),  that 

24  cubic  in.  (vol.  at  29  in.)  :  x  (vol.  at  30  in.) : :  ™  :  -QTT 

UO          OU« 

29 
therefore:     x  =  24  cubic  in.  X  gn  =23.  2  cub.  inches 

which  is  the  volume  the  gas  would  occupy  at  the  standard  pressure  of  30 
inches.  If  the  level  of  the  confining  liquid  should  not  be  the  same  inside 
the  tube  as  outside,  but  for  instance  higher,  this  column,  being  supported 

71 


72  BOYE'S   INANIMATE  MATTER. 

by  the  atmospheric  pressure,  must  of  course  be  deducted  from  its  pressure 
on  the  gas.  Thus,  suppose  the  confining  liquid  to  be  water,  the  volume  of 
the  gas,  as  before,  24  cubic  inches,  and  the  barometer  29  inches,  but  the 
water  inside  the  tube  2.9  inches  higher  than  outside.  By  dividing  the 
latter  by  13.6  (the  specific  gravity  of  mercury),  we  find  this  column  of 
water  to  be  equivalent  to  0.21  inch  of  mercury,  which,  deducted  from 
the  observed  atmospheric  pressure  of  29  inches,  leaves  28.79  inches  of  mer- 
cury, as  the  pressure  on  the  gas;  24  cubic  inches,  at  28.79  inches'  pres- 
sure, are  then  reduced  to  the  standard  pressure  of  30  inches  as  above,  by 
multiplying  by  the  former  (the  observed  pressure),  and  dividing  by  the 

28.79 
latter  (the  standard),  =  24  cubic  inches  X    ~gQ—=  23. 032  cubic  inches. 

In  the  latter  case,  however,  where  a  gas  is  measured  over  water  as  confining 
liquid,  the  thus  obtained  volume  includes  the  portion  of  vapor  of  water, 
which  is  always  formed  by  evaporation  and  adds  its  volume,  which  depends 
on  the  temperature,  to  that  of  the  gas.  To  avoid  this  error,  it  is  only 
necessary,  in  reducing  the  observed  volume  to  the  standard  pressure,  to 
deduct  from  the  atmospheric  pressure  also  that  portion  of  it,  which  is  sus- 
tained by  the  tension  of  the  vapor,  and  which  is  obtained  by  taking  from 
Table  IX,  the  maximum  tension  of  vapor  of  water  corresponding  to  the 
observed  temperature  of  the  gas.  Thus,  suppose  in  the  above  case,  the 
temperature  of  the  gas  to  be  79°  Fah.,  we  then  find  from  Table  IX  that 
the  maximum  tension  of  vapor  of  water  corresponding  to  this  temperature, 
is  0.99  inch.  From  the  whole  pressure  of  the  atmosphere,  29  inches,  we 
then  deduct,  not  only  as  before,  the  portion  sustained  by  the  column  of 
water  above  the  level  outside,  equivalent  to  0.21  inch  of  mercury,  but  also 
that,  sustained  by  the  tension  of  the  vapor,  0.99  inch,  which  thus  leaves 
only  29 — 0.21  —  0.99  =  27.80  inches  as  the  real  pressure  on  the  gas. 
The  volume  of  this,  without  the  vapor  of  water,  at  30  inches,  will  there- 

27.80 
fore  be  =  24  cubic  in.  X    gQ    =22.24  cub.  inches. 

This  volume  must  then  also  be  reduced  to  the  standard  temperature  (see 
Thermics,  under  Expansion  of  Gases),  which  is  assumed  in  England  at  60°, 
but  in  most  other  countries  at  32°.  This  is  done  by  multiplying  the 
volume  of  the  gas  by  1  +  0.002178  X  Stand.  Temp.,  and  dividing  it 
by  1  +  0.002178  X  Obs.  Temp*  Thus,  for  the  above  22.24  cub.  in.  of 

*  If  32°  Fah.  be  adopted  as  the  standard  Temp.,  the  reduction  to  this  from  any  higher 
degree  t  is  more  conveniently  performed  by  dividing  the  volume  by  1  -}-  0.00203611  (t  — 
32°) ;  the  coefficient  of  expansion  for  1°  Fah.  referred  to  the  volume  at  32°  as  unit  being 

22.24  cub.  in. 
0.00203611.    Thus,  in  this  case:  1  ,   Q  QQ203611  (79°  —  32Q)==:  2Q>298  cub< 

72 


PNEUMATICS. 


73 


79°  temperature,  we  have  its  volume  at  the  standard  temperature  of  32° 
14-0.002178  X32 


=  22.24  cub.  in. 


=2(X298  Cub'  in' 


i  +  o.002178  X  79° 
For  the  true  volume  V,  of  a  gas,  we  thus  have  the  following  formula, 

b      1+0.  002178  X^7 
V±  X  jf  X  i_|_o.002178x* 

F±  being  the  observed  volume;  5r=the  true  (77)  barometric  pressure, 
with  deduction,  if  necessary,  for  any  inequality  in  the  level  of  the  confining 
liquid  and  for  admixture  of  vapor  of  water  ;  B=  the  standard  barometric 
pressure  to  which  it  is  to  be  reduced  ;  t  =  the  temperature  of  the  gas  ; 
T=  the  standard  temperature  to  which  it  is  to  be  reduced;  and  0.00217802 
=  the  expansion  for  1°  Fah.  referred  to  the  volume  at  0°  as  unit. 


Having  thus  reduced  the  volume  of  the  gas  to  the 
standard  pressure  and  temperature,  its  weight  is  then 
easily  obtained,  if  it  be  atmospheric  air,  by  multiplying 
the  number  of  cubic  inches  thus  found,  by  the  weight 
of  1  cubic  inch  of  atmospheric  air  of  the  same  stand- 
ard pressure  and   temperature,  and  which  has   been 
given  in  57.     If  the  gas  be  any  other  than  atmospheric 
air,  we  obtain  its  weight  by  multiplying  the  weight, 
thus  found  for  atmospheric  air,  by  the  specific  gravity 
of  the  gas,  referred  to  atmospheric  air  as  a  unit,  see 
58.     Thus,  if  the  above  20.298  cub.  in.  be  Nitrogen, 
we  have  its  weight  : 
=  20.298  cub.  in.  X  0.325868  grs.  X  0.97137 
=  6.425  grains. 

Experiments  to  prove  Mdriotte's  Law. 

Fig.  40. 
30- 

c. 

-e 
-a 

7 

of  the  atmosphere  and  the  means  of  estimating  it,  we 
may  again  revert  to  the  compressibility  and  elasticity 
of  gases,  and  describe  the  experiments,  by  which  the        , 
law  already  stated  in  44  was  established  by  its  dis- 
coverer, Mariotte,  after  whom  it  has  been  called  Mari-       £- 
otte's  Law.     He  enclosed  a  quantity  of  air  in  a  tube        , 
bent  as  the  letter  J,  or  as  it  is  technically  termed,  in 
the  shape  of  an  inverted  syphon,  see  fig.  40,  the  short        *  ~ 
limb  of  which  was  sealed  and  graduated  into  volumes, 

\ 

10- 

-a 

but  the  long  one  left  open  and  furnished  with  a  scale  measuring  inches. 
Mercury  was  then  poured  into  the  open  end,  so  as  to  fill  the  bend  to  1, 

73  7 


74 


BOYE'S   INANIMATE   MATTER. 


thereby  enclosing  a  certain  volume  of  air  in  the  short  limb,  without  its  stand- 
ing with  a  higher  level  in  the  open  limb.  Under  these  circumstances,  the 
enclosed  air,  the  volume  of  which  we  will  call  1,  is  only  under  the  ordi- 
nary atmospheric  pressure,  say  30  inches  of  mercury.  More  mercury  was 
then  poured  gradually  into  the  open  limb,  by  which  the  air  in  the  closed 
limb  became  more  and  more  compressed.  The  height  of  the  mercury  in 
the  open  limb,  above  its  level  in  the  closed  limb,  was  then  carefully 
observed,  and  compared  with  the  corresponding  volume  of  the  air  in  the 
closed  limb  itself.  It  was  thus  found,  that  when  the  air  was  reduced  to 
f  of  its  original  volume,  the  height  of  the  mercury  in  the  open  limb 
above  its  level  in  the  short  limb,  from  a  to  a,  measured  10  inches,  to  which 
must  be  added  the  ordinary  atmospheric  pressure  of  30  inches,  in  order 
to  obtain  the  whole  pressure  on  the  gas,  making  it  equal  to  40  inches  of  mer- 
Fig.  42.  curv  or  1  £  =|  Atmosphere's  pressure  (61). 
More  mercury  was  then  poured  into  the  open 
end,  till  the  volume  of  the  air  was  reduced 
to  J,  when  the  height  of  the  mercurial 
column,  from  I  to  blt  producing  this  eiFect, 
was  found  to  be  30  inches,  or  1  Atmo- 
sphere, which,  added  to  the  pressure  of  the 
atmosphere  itself,  made  the  pressure  on  the 
enclosed  air  2  Atmospheres.  In  the  same 
manner,  the  column  c  c±,  when  the  volume 
was  reduced  to  J,  was  found  to  be  90  inches, 
which  being  3  Atmospheres,  added  to  the 
pressure  of  the  atmosphere  itself,  made  the 
pressure  on  the  gas  4  Atmospheres.  The 
different  volumes  of  the  air  were  thus  found 
to  be  as  1 :  f :  \ '-  i,  while  the  pressures  corres- 
ponding to  them,  were  as  1  Atmos.  :  | :  2  : 4, 
that  is,  the  volumes  occupied  by  the  air  were 
inversely  proportional  to  the  pressures  on  it. 
102.  To  prove  the  same  law  for  smaller 
pressures  than  one  Atmosphere,  a  graduated 
straight  tube,  see  Jig.  41,  open  at  its  lower 
extremity,  and  furnished  with  a  screw-stop- 
per at  its  upper  extremity,  is  immersed  with 
its  open  end  into  a  deep  glass  jar  containing 
mercury,  until  only  a  certain  known  volume 
of  air  is  left  at  its  upper  end.  This  volume  we  will  call  1.  The  tube 

74 


PNEUMATICS.  75 

being  yet  open,  and  the  mercury  having  the  same  level  inside  and  outside, 
this  volume  of  air  must  of  course  be  under  the  same  pressure  as  the  rest 
of  the  atmosphere,  that  is,  under  1  Atmosphere's  pressure.  The  tube  is 
then  closed  and  raised  out  of  the  mercury,  until  the  volume  of  the  enclosed 
air  is  increased  to  double  its  former  volume,  see  fig.  42.  The  mercury 
will  then  be  found  to  stand  much  higher  inside  the  tube  than  the  level 
a  outside  it  in  the  jar.  This  height,  from  a  to  2,  is  then  measured,  and 
will  be  found  to  be  15  inches,  which,  being  supported  by  the  atmosphere, 
must  of  course  be  deducted  from  the  ordinary  atmospheric  pressure  of 
30  inches,  in  order  to  obtain  the  pressure  on  the  gas  in  the  tube,  which, 
therefore,  will  be  30  —  15  =  15  inches  of  mercury,  =  \  Atmosphere. 
The  tube  may  then  be  raised  still  higher  out  of  the  mercury,  until  the 
enclosed  air  acquires  4  times  its  original  volume,  when  the  height  of  the 
mercurial  column,  raised  above  the  level  outside,  will  be  found  to  be  22* 
inches,  which  deducted  from  the  atmospheric  pressure  of  30  inches,  leaves 
of  this  only  7  5-  inches  or  \  Atmosphere,  as  the  pressure  on  the  gas.  We 
thus  find  in  these  experiments,  the  volumes  of  the  enclosed  air  to  be  as  1  : 
2  :  4,  while  the  pressures  are  as  1  Atmosphere  :  \  :  i,  or,  as  before,  the 
volumes  are  inversely  proportional  to  the  pressures. 

103.  A  tube  similar  to  any  of  the  above,  closed  at  one  end,  and  con- 
taining a  portion  of  air  confined  by  mercury,  is  often  designated  by  the 
name  of  a  Mariotte's  tube. 

104.  The  above  experiments  have  since  been  extended  with  atmospheric 
air  from  ^-J^  Atmosphere's  pressure  to  that  of  27  Atmospheres  (139)  and 
more,  and  Mariotte's  law  confirmed  to  this  extent.     But  it  has  also  been 
found,  that  this  law  strictly  applies  only  to  permanent  gases,  and  to  such  lique- 
fiable  gases  as  are  remote  from  their  point  of  liquefaction,  but  that  as  soon 
as  they  approach  the  latter,  their  volume  will  diminish  by  increased  pres- 
sures in  a  somewhat  greater  ratio.     This  has  been  found  to  be  the  case 
with  Sulphurous  acid  and  several  others.     In  the  same  manner,  even  Car- 
bonic acid,  if  cooled  to  32°,  has  been  found  to  expand  by  diminished  pres- 
sures more  than  it  ought  according  to  Mariotte's  law,  or  more  than  atmo- 
spheric air  does.     This  is  probably  also  the  reason,  why  most  compound 
liquefiable  gases  and  vapors  are  found,  by  experiments,  to  have  a  some- 
what greater  specific  gravity  than  that  calculated  from  the  volumes  of  their 

^-v    component  ingredients. 

LV^S       ^ ^  Pressure- Gauges. 

105.  Instruments  on  the  principle  of  the  Barometer  or  Mariotte's  tube, 
are  often  used  for  measuring  the  tension  and  elasticity  of  gases,  or  the  pres- 
sure which  they  exercise  when  confined  (see  117).      Such  instruments 

75 


76 


BOYE'S  INANIMATE   MATTER. 


L 


are  called  Pressure- Gauges,  sometimes  Manometers  (see  note  to  91).    Fig. 

43  shows,  on  an  enlarged   scale,  the    Mercurial  Exhaustion-Gauge,  m, 
Fig.  43.         attached  to  the  double-barrelled  Exhausting  Air  Pump, 

fig.  6,  to  indicate  the  quantity  of  air  remaining  at  any  time 
during  the  exhaustion,  by  the  tension  or  pressure  which  it 
exercises,  and  to  which  its  quantity  is  proportional.  It  will 
be  seen,  that  it  is  an  abridged  or  shortened  syphon  baro- 
meter, which  is  enclosed  in  a  small  separate  receiver,  con- 
nected with  the  passage  leading  from  the  barrels  a  and  b 
fig.  6  to  the  large  receiver  h.  From  an  inspection  of  fig. 
43,  it  will  easily  be  seen,  that  the  closed  limb,  being  only 
12  inches  long,  will  exhibit  no  Torricellian  vacuum,  but 
remain  filled  with  mercury,  to  the  top,  until  the  tension  or 
pressure,  which  the  air  in  the  receiver  is  capable  of  exer- 
cising on  the  mercury  in  the  open  limb,  is  reduced  to  12 
inches  of  mercury,  and  therefore  the  density  of  the  air  is 
only  J§  or  f  of  its  original  density,  f  having  been  removed ; 
after  which  all  further  rarefaction  will  be  indicated  by  it, 
the  amount  of  air  remaining  at  any  time,  being  given  as  a 
fraction,  which  has  for  its  numerator  the  mercurial  column 
sustained  by  it  in  the  gauge,  and  which  is  measured  by  the 
perpendicular  height  between  the  levels  of  the  mercury  in 
the  two  limbs  as  stated  in  65,  and  for  the  denominator  the 
whole  atmospheric  pressure  as  indicated  by  the  barometer 
at  the  time,  and  which  may  be  assumed  at  30  inches.  Thus, 
when  the  gauge  indicates  10  inches  as  in  the  figure,  the  re- 
maining air  is  -JJJ  =  -J  of  its  original  amount,  and  when  the 

A 

gauge  indicates  -f^  inch,  the  remaining  air  is  ^  =  -3^.  ^ 

106.  For  measuring  pressures  larger  than  the  ordinary  atmospheria 
pressure,  Mercurial  Pressure-Gauges  receive  the  forms  represented  in  Jigs. 

44  and  45.     Fig.  44  has  the  general  form  of  a  cistern  barometer,  but  the 
cistern  c  containing  the  mercury  is  closed  air-tight  at  the  top  and  made 
to  communicate  with  the  vessel,  in  which  the  gas  is  confined,  by  a  small 
tube,  passing  from  the  top,  or  as  a  fig.  44,  through  the  bottom  to  above 
the  level  of  the  mercury.     The  tube  b  is  open  at  the  upper  end,  and  the 
pressure,  therefore,  estimated  by  the  height  of  the  column  of  mercury, 
which  is  forced  up  in  it,  for  which  purpose  it  is  furnished  with  a  scale 
measuring  inches,  2  inches  being  equivalent  to  1  pound  on  the  square  inch. 
Fig.  45  exhibits  another  pressure-gauge,  which  is  easily  constructed  out  of 


PNEUMATICS. 


77 


Fig.  45. 


20,, 


a  glass  tube  by  bending  it  twice.    The  pressure  is  measured  by  the  difference 
Fig.  44.  between  the  two  levels  of  the  mercury  in  the  two 

limbs  (65).  For  measuring  very  small  pressures, 
such  as  that  under  which  ordinary  lighting  gas  is 
forced  through  the  burners  from  the  pipes,  it  is  made 
to  contain  water  instead  of  mercury,  in  which  case 
for  great  accuracy  the  tube  should  be  i  inch  in  diani. 
and  each  limb  furnished  with  a  vernier.  As  the  tube 
of  this  kind  of  pressure-gauges  is  open  towards  the 
atmosphere,  and  the  mercurial  column  in  it,  there- 
fore, subject  to  the  atmospheric  pressure,  it  is  neces- 
sary, in  order  to  obtain  the  whole  tension  or  elasticity 
of  the  confined  gas,  to  add  to  the  above  pressures  indi- 
cated by  the  mercurial  column  in  the  gauges,  the 
ordinary  atmospheric  pressure,  but  this  is  often 
omitted,  and  the  pressure  only  given  as  being  over 
and  above  the  outer  atmospheric  pressure. 

107.  The  above  mercurial  gauges,  in  which  the 

pressure  is  measured  by  the  height  of  the  column  of  mercury,  which  it  can 
sustain,  are  the  most  reliable  of  all,  but  they  have  the  serious  inconvenience, 
that  when  the  pressure  becomes  large,  for  instance  in  high-pressure  steam- 
boilers,  where  it  often  exceeds  60  pounds  to  the  square  inch,  or  4  Atmospheres, 
the  tube  must  be  more  than  4  X^O  in.  =10  feet  long  (see  139).  For  such 
Fig.  46.  high  pressures  Condensed  Air  or  Mariotte's  Tube  Gauges  are 
often  substituted,  acting  on  the  principle  of  estimating  the  pres- 
sure from  the  volume  of  a  confined  portion  of  air.  Any  of  the 
above  gauges  j%s.  44  and  45  may  be  converted  into  such  by  closing 
the  upper  end  of  the  tube,  so  as  to  confine  the  portion  of  atmo- ' 
spheric  air  which  is  containe'd  in  it,  which  volume  is  then 
divided  into  fractions.  Fig.  46  exhibits  a  gauge  of  this  kind, 
such  as  is  used  by  gas-fitters  to  prove  by  high  pressure  the 
tightness  of  gas  pipes.  For  small  pressures  the  tube  is  left 
open  at  the  top,  and  it  then  acts  as  one  of  the  above-described 
mercurial  gauges.  When  used,  it  is  screwed  on  the  end  of 
one  of  the  pipes,  into  which  air  is  forced  by  a  forcing  pump. 
Any  leakage  is  indicated  by  the  gradual  diminution  of  the  pres- 
sure. For  convenience  in  the  making  of  it,  the  cistern  c  is  made  of 
brass ;  but  as  this  is  corroded  by  mercury,  the  latter  is  contained  in  an 
iron  cup  i,  placed  inside.  The  cover  into  which  the  tube  b  is  cemented, 
is  made  to  screw  on  air-tight.  The  compressed  air,  the  elasticity  of  which 

77 


15.  a!*. 


/  *£„ 


78 


BOYE'S  INANIMATE  MATTER. 


X- 


we  want  to  measure,  finds  its  way  between  the  cup  i  and  the  inside  of  the 
cistern  c,  so  as  to  press  on  the  top  of  the  mercury,  which,  being  forced  up 
into  the  tube  b  closed  at  the  tipper  end,  will  compress  the  atmospheric  air 
which  it  contains,  from  the  volume  of  which  the  pressure  is  ascertained. 
Thus,  when  its  volume  is  reduced  to  £,  the  pressure  on  it  is  2  Atmospheres, 
or  1  Atmosphere  over  the  ordinary  atmospheric  pressure ;  when  compressed 
to  },  the  pressure  is  3  additional  Atmospheres  over  the  ordinary  atmo- 
spheric pressure ;  when  compressed  to  $,  7  additional  Atmospheres.  To 
these  pressures  must,  however,  be  added,  in  order  to  find  the  pressure  or 
elasticity  of  the  confined  gas,  which  we  want  to  measure,  the  column  of 
mercury  inside  the  tube  above  the  level  in  the  cup  i.  Thus,  if  the  height 
of  this  be  6  inches,  when  the  volume  is  J,  the  elasticity  of  the  confined  gas 
is  g^j  =  -J  Atmosphere  more  than  indicated  by  the  volume  of  the  air  in 
the  tube,  or  altogether  1-j-J  Atmosphere,  =  36  inches  of  mercury,  or  18 
pounds  to  the  square  inch;  if  9  inches,  when  the  air  is  compressed  to  £, 
Fig.  47.  the  whole  pressure  is  3-}~39o  Atmospheres;  if  10 £  inches, 

10J 

when  compressed  to  J,  7-f  gQ~  — 7  gj  Atmospheres,  &c. 

It  is  a  matter  of  course,  that  if  the  temperature  be  not 
constant,  its  effect  on  the  confined  air  in  the  tube  must  also 
be  taken  into  consideration,  by  first  reducing  its  volume  to 
the  same  temp.,  as  in  100. 

108.  Condensed  air  pressure-gauges,  besides  being 
considerably  affected  by  the  temperature,  have  also  the 
great  objection,  that  as  the  pressure  increases,  and  it  in 
many  cases  becomes  important  to  estimate  it  with  increased 
accuracy,  the  divisions  of  the  scale,  corresponding  to  the 
same  increase  in  pressure,  diminish  very  rapidly  in  size, 
and  thus  become  less  accurate.  This  latter  may,  how 
ever,  be  partly  remedied  by  furnishing  the  gauge  with  two 
tubes,  see  fig.  47,  as  first  contrived  by  Dr.  J.  K.  Mitchell 
in  his  experiments  on  the  liquefaction  of  carbonic  acid. 
The  second  tube  b  is  enlarged  at  the  end  which  dips  into 
the  mercury,  by  being  cemented  into  a  short  iron  tube  d 
of  larger  diameter,  which  forms  its  lower  extremity  and  the 
capacity  of  which  is  such,  that  the  mercury  only  enters 
the  glass  tube  ate,  when  the  pressure  approaches  that  which 
we  particularly  want  to  measure.  Thus,  suppose  that  the 
mercury  in  d  only  reaches  to  e,  when  the  air  in  a  is  com- 
pressed to  |  its  original  volume,  and  that  then  the  mercurial  column  in  it  is  36 

78 


-13 


PNEUMATICS.  79 

inches  above  the  level  at  c.  The  pressure  measured  will  then  be  9! 
Atmospheres.  Deducting  from  this  the  column  from  c  to  e,  the  pressure 
on  the  air  in  the  tube  b  will  be  exactly  9  Atmospheres.  If  this  volume  of 
the  tube  above  e  be  divided  into  fractions,  it  is  evident  that  when  the 
enclosed  air  is  reduced  to  J  of  this  volume,  the  pressure  on  it  will  be  18 
Atmospheres,  and  when  reduced  to  i,  36  Atmospheres;  to  which,  of  course, 
in  order  to  obtain  the  pressure  we  want  to  measure,  must  be  added  the  mer- 
curial column  beyond  e. 

109.  For  experiments  on  a  small  scale,  as  for  the  compression  of  gases 
in  glass  tubes,  a  capillary  tube  of  the  proper  length,  see  a  b  fig.  48,  is 

Fig.  48.  employed   as   a   gauge,  having   no  cistern. 

^_  -4-        £  Jk-tc^  Being   closed    at   one   end    at  a,    a   small 

&  c  'a  column   of  mercury   c  is    introduced   into 

the  other  open  end  b,  by  expelling  from  it,  by  heat,  the  smallest  possible 
quantity  of  air,  and  then  dipping  the  open  end  into  mercury,  till  on 
cooling  a  small  quantity  of  this  is  drawn  into  it  (38),  which  then  con- 
fines the  air  remaining  in  the  tube.  The  space  ac  occupied  by  this  air  is 
then  divided  as  before  into  fractions  of  its  own  volume.  When  using  it, 
the  open  end  b  is  either  cemented  into  a  metallic  socket,  which  is  screwed 
on  to  the  end  of  the  tube  in  which  the  gases  are  compressed,  or  in  some  cases 
the  whole  gauge-tube  may  be  slipped  into  the  compression-tube,  in  which 
case  no  strength  is  required  of  its  sides,  and  these  may  therefore  be  of  any 
thinness,  and  the  whole  gauge,  therefore,  of  miniature  dimensions.  If  this 
gauge  be  in  a  horizontal  position,  no  allowance  whatever,  need  be  made  for 
the  weight  of  the  mercurial  column  c;  and  the  volume  of  the  confined  air, 
therefore,  indicates  the  whole  tension  of  the  gas  which  we  want  to  measure. 

110.  Gauges  for  measuring  high  pressures  are  particularly  required  for 
high-pressure  steam-boilers,  to  indicate  at  any  time  with  accuracy  the  ten- 
sion or  elasticity  of  the  steam,  and  thereby  to  warn   against  accidents. 
Such  gauges  are  called  Steam- Gauges,  sometimes  also  Manometers,  see 
foot-note   to  91.      Besides   the  before   described   pressure-gauges,  many 
others  have   been   constructed  for   steam-gauges  on    different  principles. 
Thus,  the  principle  of  Bourdon's  Metallic  Barometer  (89),  was  first  employed 
for  a  steam-gauge,  by  admitting  the  steam  into  its  hollow  hoop-like  vessel. 
In  the  same  manner  an  accurate  thermometer  will  indicate  from  the  temp, 
of  the  steam,  its  pressure,  see  138  &c.     A  number  of  steam-gauges  act  on 
the  principle  of  letting  the  steam  act  on  a  metallic  valve,  so  as  to  compress  a 
spring  (Spring-Gauges),  or  raise  a  known  weight.     These  are,  however,  not 
so  much  for  the  purpose  of  measuring  the  pressure  of  the  steam  as  for  afford- 
ing escape  and  safety  from  it,  when  its  elasticity  should  exceed  a  certain 
limit,  and  they  are  therefore  called  Escape  or  Safety  Valves,  see  146  Jig.  70. 

79 


80 


BOYE'S  INANIMATE  MATTER. 


iments  to  illustrate  the  pressure  of  the  Atmosphere. 


Fig.  50. 


..  The  atmospheric  pressure  on  the  surface  of  liquids,  may  be  illus- 
trated by  the  Fountain  or  Jet  in  Vacuo,  see  fig.  49,  which  consists  of 
Fig.  49.  a  closed  receiver,  which  is  furnished 

at  its  lower  extremity  with  a  stop-cock 
c,  from  which  a  jet  projects  into  the 
receiver,  terminating  outside  by  a  screw 
s,  by  which  it  may  be  attached  to 
the  air-pump.  Having  exhausted  the 
receiver,  it  is  detached  from  the  air- 
pump,  and  the  mouth  of  the  stop-cock 
immersed  into  a  vessel  containing  water. 
On  opening  the  stop-cock  the  atmo- 
spheric pressure  will  force  the  water 
in  a  jet  into  the  exhausted  receiver. 

112.  The   Mercurial  Ham,  see  jig. 
50,  is  used  to  illustrate  the  same  in 
connection  with  the  porosity  of  certain 
substances  such  as  wood,  leather,  &c. 
It  consists  of  a  receiver  having  inserted 
in    the  top  a  cup  c,  which  is   closed 
at  the  bottom  by  a  stopper  of  wood  cut 
across  the  grain,  or  by  a  piece  of  buck- 
skin, and  which  contains  mercury.     On 
exhausting  the  receiver,  the  atmospheric 
pressure  will  force  the  mercury  through 
the  pores  in  small  globules  as  a  rain. 
113.  If  a  piece  of  thin  bladder  be  tied  over  the  top  of  a  small  wide- 
mouthed   receiver,   the  Bladder   Glass,    see  fig.  51,  and   the    air  then 
Fig.  51.         quickly  exhausted,  the  atmospheric  pressure  will  burst  the 
bladder  inward  with  a  loud  report  as  from  an  explosion. 

114.  The  pressure  on  any  part  of  an  elastic  fluid  being 
equally  communicated  to  all  parts  of  it,  it  is  evident  that 
the  pressure  which  it  in  return  exercises  on  all  the  con- 
fining limits  must  be  uniform,  and  must,  therefore,  also 
extend  to  the  whole  surface  of  any  object  immersed  in  it. 
The  direction  of  its  pressure  at  any  point  of  all  such  sur- 
faces, is  always  in  the  perpendicular  to  them  at  that  point.  The  Upward 
Pressure  of  the  atmosphere  on  an  under  surface,  may  be  illustrated  by 

80 


PNEUMATICS. 


81 


a  syringe  with  a  solid  piston,  see  fig.  52.  Having  drawn  the  piston  out  and 
Fig.  52.  attached  a  weight  to  the  piston-rod,  suspend  it,  and  con- 
'  nect  the  upper  extremity  of  the  barrel  by  the  tube  a  with 
an  exhausting  air-pump.  When  the  air  is  exhausted  from 
it,  the  atmospheric  pressure,  acting  perpendicularly  upward 
on  the  lower  surface  of  the  piston,  will  force  it  up,  thereby^ 
raising  the  weight  attached  to  the  piston-rod. 

115.  The  same  may  be  illustrated  by  the  Magdeburg 
Hemispheres,  which  are  two  hollow  hemispheres,  see  a  and 
b  fig.  53,  having  their  edges  ground  true,  so  as  to  fit  air- 
tight together,  thus  forming  a  hollow  sphere.  One  of  them 
is  furnished  with  a  handle,  and  the  other  with  a  stop-cock 
and  a  screw  c,  by  which  it  may  be  attached  either  to  an 
air-pump,  or  to  a  handle  d.  If  the  two  hemispheres  be 
put  together,  and  the  air  inside  exhausted,  the  pressure 
of  the  atmosphere  outside  will  force  them  together,  so 
that  if  they  be  removed  from  the  air-pump,  and  the  handle 
attached,  it  will  require  a  considerable  force  to  separate 
them.  To  calculate  the  exact  force  with  which  they  are 
held  together,  it  must  be  remembered,  that  though  the 
whole  atmospheric  pressure  on  them  is  equal  to  15  pounds 
on  each  square  inch  of  the  whole  outer  surface,  it  is  only 
that  portion  of  it  which  acts  at  right  angles  to  the  plane 
of  the  joint,  which  holds  them  together.  Thus,  if  the 
radius  of  the  sphere  be  2  inches,  the  plane  surface  of  the 
circular  joint  (r2  TT)  will  be  =  2a  X  3?  =  12|  square 
inches,  and  the  pressure  on  it,  therefore,  124  x  15  Ibs. 


Fig.  53. 


=188  1  Ibs.  To  pull  them  apart,  this  force  must,  therefore, 
be  applied  from  each  side.  They  have  received  their  name 
from  the  fact,  that  they  were  first  contrived  by  Otto  von 
Guericke,  Burgomaster  of  Magdeburg,  a  town  in  Germany, 
who  in  1650  had  invented  the  Air-Pump.  To  illustrate 
the  Atmospheric  pressure,  he  exhibited,  in  1654  at  Regens- 
burg,  to  the  Emperor  Charles  Y,  in  presence  of  the  Imperial 
Diet,  a  pair  of  these  of  about  two  feet  in  diameter,  to  which 
twenty-four  horses  were  attached,  without  their  beina^rt>Je  to 


116.  The  external  surface  of  the  human  body  being  about 
2000  square  inches,  it  is  evident  that  it  must  be  exposed  to 
a  pressure  from  the  atmosphere,  of  about  30.000  Ibs.,  or  nearly  14  tons.    That 
F  81 


82  BOYE'S   INANIMATE   MATTER. 

this  pressure  does  not  force  in  the  Abdominal  and  Thoracic  cavities  of  the 
body,  is  prevented  by  the  access  of  the  atmosphere  to  them,  by  which  the 
external  and  internal  pressures  are  counteracted.  The  solid  walls  of  the  body 
forming  them,  are,  however,  subject  to  it;  these  and  the  internal  organs 
are,  however,  prevented  from  being  crushed  by  it  on  account  of  the  uni- 
'  formity  of  the  pressure,  by  which  the  particles,  being  pressed  equally  on  all 
sides,  have  no  tendency  to  change  their  relative  position,  crushing  being 
merely  produced  by  an  unequal  pressure.  This  is  also  the  reason  why  we 
are  not  conscious  of  its  existence.  It  may,  however,  easily  be  made  mani- 
fest by  removing  the  pressure  from  any  part  of  the  body,  for  instance,  by 
Fig.  54.  placing  the  hand  over  the  mouth  of  a  small 

receiver,  see  fig.  54,  and  exhausting  the  air 
from  within  it;  the  pressure  on  the  opposite 
side  of  the  hand  will  then  force  it  against  the 
edge  of  the  receiver  and  cause  those  parts,  from 
which  the  pressure  is  removed,  to  bulge  into 
it.  The  operation  of  cupping  depends  on  this 
^  same,  for  if  small  cuts  be  previously  made 

k^j        i»»^  through  the  skin,  the  blood  will  be  forced  out 

through  them  by  the  pressure  on  the  rest  of 
the  body.  In  such  places,  however,  of  the  body,  where  the  parts  are  not 
soft  or  permeable  to  fluids,  this  pressure  is  used  by  nature  to  sustain  and 
keep  together  its  different  parts,  without  calling  into  requisition  for  this 
purpose  the  power  of  the  muscles.  Thus  all  the  movable  joints  of  the 
body  are  kept  together  by  the  articulating  surfaces  of  the  bones  being  sur- 
rounded by  an  air-tight  ligament,  so  that  they  may  slide  freely  over  each 
other,  but  cannot  be  separated  without  producing  a  vacuum,  and  are  thus 
forced  together  by  the  atmospheric  pressure,  amounting,  for  instance,  on 
the  knee-joint  to  upwards  of  100  Ibs.  By  actual  experiment,  by  dissect- 
ing away  from  the  hip-joint  every  thing  excepting  the  capsular  ligament, 
and  suspending  it  under  a  pneumatic  receiver  with  a  weight  attached  to 
the  thigh-bone;  this  has  been  found  to  drop  out  of  the  socket,  on  exhaust- 
ing the  air  from  the  receiver,  but  to  return  again  into  it,  on  re-admitting 
the  air  The  excessive  fatigue  experienced  in  ascending  high  mountains, 
has  been  ascribed  to  the  diminution  of  the  atmospheric  pressure,  by  which 
the  weight  of  the  limbs  has  to  be  in  part  supported  by  the  muscles, 
instead  of  by  the  atmospheric  pressure  alone. 

f\        £ Experiments  to  illustrate  the  Expansibility,  Elasticity  and  Compressi- 
J  *  &  $$'^  bility  of  Atmospheric  Air. 

111.  Expansibili^and  Elasticity  of  gases  both  depend  on  the  same 

82 


PNEUMATICS. 


83 


repulsive  action  between  the  atoms,  which  we  have  called  negative  cohe- 
sion (12),  and  which  causes  them  to  have  a  constant  tendency  to  extend 
their  volume  and  thereby  to  exercise  a  certain  pressure  on  the  confining 
limits,  which  these  must  return,  in  order  to  restrain  them ;  and  as  soon  as 
this  restraining  pressure  is  diminished  or  ceases,  it  causes  them  actually 
to  extend  their  volume.  They  are  therefore  in  fact  the  same  property,  but 
the  word  elasticity  is  only  applied  to  their  expansive  force  after  a  previous 
diminution  of  their  volume,  by  an  increase  of  the  pressure  of  the  confining 
limits,  while  for  their  expansive  force  under  the  ordinary  atmospheric 
pressure,  or  after  its  diminution,  the  word  tension  is  generally  used. 


Fig.  55. 


Fig.  56. 


118.  The  Expansibility  of  atmo- 
spheric air  is  illustrated  by  forcing 
the  greater  portion  of  the  air  out 
of  a  sound  bladder  or  small  gum- 
bag   by   compression,    and   then 
closing   the    orifice   by  tying   it 
firmly  with  a  string.      Place   it 
under  a  receiver  as  in  fig.  55.    As 
soon  as  the  air  is  exhausted  from 
the  receiver  outside  the  bladder, 
the   small   quantity  of  air  con- 
tained inside  it  will  expand,  and 
swell  the  bladder  out,  as  seen  in  fig.  56.     When 
the  air  is  again  admitted  into  the  receiver,  the 
bladder  will  collapse  to  its  former  dimensions. 
The   same  experiment  will  often  succeed  with 
dried  and  shrivelled  fruit,  as  raisins,  which,  if  the 
skin  be  sound,  will,  in  a  similar  manner,  be  blown 
out  to  their  original  fullness  by  the  small  quantity 
of  air  which  they  contain. 

119.  Mechanism  of  Respiration.  It  is  by  a 
similar  contrivance  that  air  is  made  to  enter  into 
the  lungs  by  respiration.  The  lungs  may  be  con- 
sidered as  two  membranous  bags,  only  divided  into 
a  number  of  smaller  compartments  or  cells,  but 
all  communicating  with  each  other  by  the  bron- 
chial ramifications,  through  which  the  air  may 
enter  into  them  by  way  of  the  mouth  and  the 
windpipe;  the  whole  apparatus  being  suspended 
in  the  cavity  of  the  chest,  as  may  be  represented  by  the  bladder  a  fig.  57, 

83 


84 


BOYE'S  INANIMATE   MATTER. 


attached  to  the  pipe  b  and  fixed  in  the  receiver  h.  The  expiration  is  effected 
by  diminishing  the  cavity  of  the  chest  by  the  contraction  of  the  ribs  and 
the  raising  of  the  diaphragm,  by  which  the  air,  in  consequence  of  its  elasticity, 
is  forced  out  through  the  windpipe  by  compression.  This  may  be  imitated 
by  blowing  the  bladder  out  through  the  pipe  I  and  closing  this  with  the 
finger,  until  the  mouth  of  the  receiver  be  immersed  into  a  vessel  e  e,  with 
water.  On  removing  the  finger  and  depressing  the  receiver  further,  the 
air  will  be  forced  out  through  the  pipe  6,  as  represented  in  fig.  57.  The 
inspiration,  on  the  contrary,  is  effected  by  enlarging  the  cavity  of  the 
chest  by  expanding  the  ribs  and  flattening  the  diaphragm,  by  which  a 
vacuous  space  is  produced  between  the  inside  of  the  chest  and  the  mem- 
brane of  the  lungs,  by  which  the  air,  in  virtue  of  its  expansibility,  will 
enter  and  innate  them.  This  operation  may  be  imitated  with  the  above 
apparatus  by  gradually  drawing  the  receiver  li  again  out  of  the  water, 
thereby  enlarging  its  capacity  and  producing  a  partial  vacuum.  The  air 
Fig.  58.  then  enters  by  its  expansibility  through  the  pipe 

&  and  inflates  the  bladder  as  in  Jig.  58. 

120.  Common   water   freshly   drawn   always 
contains   more   or    less   air    in    solution   (55). 
When  the  pressure  is  removed  from  its  surface 
by  placing  it  in  a  tumbler  under  a  receiver  and 
exhausting  the  air,  the  expansibility  of  the  dis- 
solved air  will  overcome  the  adhesion,  by  which 
it  is  kept  in  solution,  and  most  of  it  will  appear 
as  small  bubbles  on  the  sides  oL4ke  vessel  and 
escape  through  the  water.          I  jfyQj\J\$U(/S 

121.  Place  a  piece  of  charcoed  or  any  other 
porous  body  in  a  tumbler  filled  with  water,  and 
this  under  a  receiver,  and  exhaust  the  air  from 
the  latter.     The  air  contained  in  the  pores  of  the 
charcoal   will   expand   and   escape   in    bubbles 
through  the  water.     On  readmitting  the  air,  the 
atmospheric   pressure  will  force  the  water  into 
the  pores,  which   thus  will   become  filled  with 
water  instead   of   air.     This   method   is   often 

employed  to  fill  the  pores  of  other  porous  bodies  with  water.  If  the  pores 
of  wood  be  filled  in  this  manner  with  water  instead  of  air,  it  will  become 
water-logged  and  incapable  of  noatinj 

an  apparatus  useBTto  illustrate  the  compressibility, 
elasticity  and  expansibility  of  atmospheric  air.     It  consists  of  a  strong, 

84 


PNEUMATICS. 


85 


Fig.  59.  generally  spherical  vessel,  a  fig.  59,  having  a  tube  in- 

serted at  the  top,  reaching  nearly,  though  not  quite,  to 
the  bottom,  and  furnished  outside  with  a  stop-cock  and 
screw  for  attaching  a  jet  i.     A  quantity  of  water  suffi- 
cient to  close  the  end  c  of  this  tube,  is  introduced  into 
the  vessel,  either  by  unscrewing  the  tube,  or  by  remov- 
ing a  portion  of  the  air  from  it  by  suction,  and  then, 
after  having  inverted  it  and  immersed  the  jet  of  the 
tube  into  water,  opening  the  stop-cock,  when  the  atmo- 
spheric pressure  will  force  in  a  sufficient  quantity  of  the 
latter  (111).     Remove  then  the  jet  and  attach  to  it  a  condensing  syringe 
(41).     By  every  stroke  of  the  piston,  the  air  forced  into  the  vessel  will 
be   seen   to  bubble  through  the  water.     Having   closed  the   stop-cock, 
remove  the  condenser  and  replace  the  jet.     On  turning  the  stop-cock,  the 
elasticity  of  the  compressed  air  will  force  the  water  out  in  a  jet.     Having 
replenished,  if  necessary,  the  vessel  with  water,  place  it  under  a  receiver 
and  exhaust  the  air,  see  fig.  60.     By  thus  removing  the  pressure  of  the 
Fig.  60.  atmosphere  on  the  water  inside  the  jet,  the  expan- 

sibility of  the  atmospheric  air  enclosed  in  the  ves- 
sel will  force  the  water  out  in  a  jet.  Hero's  ball 
is  so  called  from  its  inventor,  who  lived  in  Alex- 
andria, and  described  this  apparatus  about  one 
hundred  and  twenty  years  before  the  Christian 
era. 

123.  Contrivances  acting  on  the  same  principle 
as  Hero's  ball,  and  called  Air-Chambers,  are 
attached  to  most  hydraulic  engines,  such  as  the 
Fire  Engine  and  the  Hydraulic  Ram,  in  order  to 
convert  the  intermitting  jet  of  these  into  a  con- 
tinuous. The  air-chamber  consists  of  a  strong, 
more  or  less  spherical  vessel  of  metal,  at  the  bot- 
tom of  which  the  water  is  forced  in  through  a  valve  faster  than  it  issues  from 
the  jet,  which  may  either  pass  from  near  the  bottom  through  the  top,  as 
in  Hero's  ball,  or,  as  is  more  common,  from  the  side  near  the  bottom.  The 
air  enclosed  in  the  chamber  is  thus  compressed  and,  by  its  elasticity,  forces 
J;he  water  out  in  a  constant  stream,,  /] 


npa^Tanddnertia  of  Gases,   f 

124.  Gases,  like  all  other  matter,  possess  Inertia;  hence  the  atmosphere 
offers  a  resistance  to  all  bodies  moving  in  it,  because  these  have  to  impart 

85  8 


86 


BOYE'S   INANIMATE   MATTER. 


to  it  by  impact  some  of  their  motion,  in  order  to  move  it  out  of  their  way. 
For  this  reason,  bodies  which  present  a  large  surface,  lose  their  motion 
sooner  than  those  which  present  a  smaller,  or  one  of  a  more  favourable 
shape.  In  the  same  manner,  specifically  light  bodies  lose  their  motion 
sooner  than  heavy  bodies,  which  within  the  same  space  contain  more 
moving  matter  and,  therefore,  more  motion.  This  may  be  illustrated  by 
the  Windmill  experiment,  which  is  performed  by  an  apparatus,  see  Jig.  61, 

having  two  axes  i  i,  perfectly  alike,  and  fur- 
nished with  small  pinions,  that  are  worked  by 
two  perfectly  similar  racks  r  r,  attached  to 
the  same  weight  w,  so  that  the  latter  by  its 
descent  imparts  exactly  the  same  velocity  to 
them  both.  At  right  angles  to  each  of  the  axes 
i  i,  are  attached  four  perfectly  similar  wings, 
which  may  be  turned  so  as  to  present  either 
their  broad  surface  or  their  edge  to  the  air, 
when  the  axes  revolve.  Place  first  the  wings 
of  both  axes,  so  as  to  present  the  broad  sur- 
face to  the  air,  when  revolving.  Let  then 
the  weight  drop  so  as  to  impart  to  them  both 
the  same  velocity.  They  will  both  stop  at 
the  same  time,  arid  soon,  on  account  of  the 
resistance  of  the  air.  Turn  then  the  wings 
of  one  axis  so  assto  present  the  edges  to  the 
air,  and  start  them  again  by  the  descent  of  the 
weight.  The  one  with  the  wings  turned 
edgeways  will  then  continue  its  motion  much 
longer  than  the  Other.  But  if  the  apparatus 
be  placed  under^  an  exhausted  receiver,  and 
the  weight  again  made  to  descend  by  the 

rod  g  g,  passing  through  a  stuffing-box  s  at  the  top  of  the  receiver,  both 
axes  will  be  found  to  continue  their  motion  equally  long  in  the  vacuum, 
r  lilthough  the  wings  of  the  one  are  turned  differently  from  those  of  the  other. 
125.  The  resistance  of  the  air  by  its  inertia,  is  the  cause  why  specifically 
lighter  bodies  fall  in  the  atmosphere  slower,  than  heavier.  In  a  vacuum 
all  bodies  fall  equally  fast.  This  may  be  illustrated  by  the  Feather  and 
Guinea  experiment,  fig.  62,  which  is  performed  by  a  tall  receiver  h,  contain- 
ing several  drop-stages  ddd,on  one  of  which  is  placed  a  gold-piece,  and  on 
another  a  feather.  The  air  being  exhausted,  these  are  allowed  to  begin 
their  descent  at  the  same  time,  by  allowing  the  stages  to  drop  simultane- 

8G 


PNEUMATICS. 


87 


ously  by  the  rod  g,  passing  through  the  stuffing-box  s.  If  the  air  is  well 
exhausted/ they  will  both  reach  the  bottom  at  the 
same  time,  showing  that  it  is  the  resistance  of  the 
air,  which  causes  the  feather  and  specifically  lighter 
bodies  in  general  to  fall  slower  than  heavier  ones. 
126.  The  resistance  on  a  sphere  of  5  inches 
diameter,  falling  through  the  air,  has  been  esti- 
mated to  be  1.211  oz.,  when  it  acquires  the  velo- 
city of  30  feet  per  second.  But  this  resistance 
is  increased  in  a  much  greater  ratio  than  the 
velocity  of  the  moving  body,  it  being  proportional 
to  the  squares  of  the  velocities  (See  under  Stereo- 
Dynamics).  For  this  'reason  rain-drops,  hail- 
stones, and  all  kinds  of  projectiles,  such  as  musket 
and  cannon  balls,  have  all  a  maximum  velocity  in 
the  air,  which  they  cannot  exceed.  But  the  larger 
their  size,  or  the  greater  the  specific  gravity  of  the 
material  of  which  they  are  made,  the  greater  is 
the  velocity  that  can  be  given  to  them.  Thus,  a 
bullet  of  lead  is  capable  of  a  greater  velocity  than 
one  of  iron.  The  flight  of  birds  depends  on  this 
same  increase  in  the  resistance  of  the  air,  the  mo- 
tion of  their  wings  being  performed,  in  one  direc- 
tion, both  with  greater  surface  and  with  greater  velocity,  than  in  the  other. 
127.  Air  which  thus  receives  motion  by  impact  or  otherwise,  will  by 
the  same  property  of  inertia  continue  its  motion,  on  which  the  operations 
of  fanning  and  blowing  depend,  until  it  in  its  turn  is  checked  by  some 
other  cause ;  for  instance,  by  striking  against  other  air,  or  against  iru-- 
movable  objects  on  the  earth.  The  performance  of  windmills  and  the 
sailing  of  ships  depend  on  motion  received  by  impact  from  moving  masses 
of  air,  which  constitute  winds.  The  power  of  winds  increases  in  the  same 
augmented  ratio  of  the  squares  of  their  velocities,  which  are  stated  to  be  as 
follows : 


Gentle  breeze. 
Pleasant  breeze. 
High  wind. 
Storm  or  gale. 
Great  storm. 
Hurricane. 


Vel.  in 
miles 
per  hour, 
i.         ...         .       3.25 

Vel.  in 
ft.  per 
second. 

4.77 

Inch,  of 
water 
supported. 
0.01 

Pressure  on 
a  square  ft. 
in  Ibs.  Avoir  d.p. 

0.83  oz. 

ze.          .        .          6.5 

9.53 

0.04 

3.33   " 

.     16.25 

23.83 

0.25 

1   Ib.  5  oz. 

.         .         .         32.5 

47.66 

1. 

5    "     3   « 

.     56.29 

82.56 

3. 

15     "     9   " 

79.61 

116.76 

6. 

31    "    3  " 

mrricane.  .        .    97.5 

143.00 

9. 

46    «  12  " 

87 

vy 

128.  The  direction  of  the  wind  is  generally  ascertained  by  the  vane, 
but  when  feeble,  by  a  suspended  silk  ribbon,  or  an  ascending  column  of 
smoke ;  and  sometimes  also  by  the  cold  experienced  on  the  finger,  when 
moistened  and  held  up  to  the  air.  The  force  of  winds  is  estimated  by 
instruments  called  Anemometers,  the  best  of  which  are  constructed  on  the 
principle  of  the  pressure-gauge  (106)  fig.  45,  being  made  of  large  diame- 
ter and  containing  water  instead  of  mercury,  having  also  the  limb,  acted 
on,  horizontal,  so  as  to  turn  it  against  the  wind.  But  those  generally 
adopted  as  the  most  convenient  in  meteorological  observatories,  are  made 
on  the  principle  of  spring-gauges,  exposing  a  surface  of  a  known  area  to 
the  action  of  the  wind,  the  pressure  on  it  being  estimated  by  the  compres- 
sion of  springs.  Such  has  been  made  self-registering  by  Osier  (L.  &  E. 
Phil.  Mag.  vol.  xi.  p.  476),  so  that,  being  connected  with  a  vane,  it  will  note 
by  a  pencil  both  the  direction  and  the  force  of  the  wind  for  every  moment. 
*—  129.  When  a  gas  is  allowed  to  escape  from  a  confining  vessel  through 
a  small  or  capillary  orifice  in  a  thin  plate  into  a  vacuum,  the  velocity  with 
which  it  issues  remains  the  same ;  for,  as  the  density  and  consequent  elas- 
ticity or  propelling  force  of  the  gas  decreases,  its  specific  gravity,  and  con- 
sequently also  the  propelled  quantity,  decreases  in  the  same  ratio,  so  that 
in  the  same  time  the  same  volume  of  gas  always  passes  out,  but  of  course 
of  constantly  diminishing  density. 

If  a  gas  be  allowed  to  flow  through  a  similar  small  orifice,  but  from  a 
vessel  in  which  it  is  kept  under  a  constant  pressure  (see  gasometers  ), 
it  will  be  found  that  the  velocity  with  which  it  flows  out  increases  rapidly, 
as  the  space  into  which  it  flows  is  rendered  more  and  more  vacuous,  until 
the  tension  of  the  remaining  air  is  only  about  £  Atm.  (10  inches  of  mer- 
cury), after  which  further  exhaustion  will  not  be  found  to  increase  the 
velocity  in  the  same  proportion,  and  when  the  state  of  rarefaction  reaches 
gLth  (1  inch  of  mercury,  see  105),  all  further  exhaustion  seems  scarcely 
to  affect  the  velocity,  if  the  pressure  on  the  gas  be  1  Atmosphere.  In  this 
manner,  in  1000  seconds,  60  cub.  inches  (15148  fluid-grain  measures)  of 
dry  atmospheric  air,  have  been  found  to  flow  into  such  a  vacuum  through 
an  orifice  in  a  platinum  foil  of  3^0^  °f  an  ^ncn  m  diameter.  The  times 
which  the  same  volumes  of  different  gases  require  for  their  passage  into 
such  a  vacuum,  have  been  found  to  vary  so  as  to  be  proportional  to  the 
square  roots  of  their  specific  gravities,  and  their  velocities,  therefore,  under 
the  same  circumstances,  to  be  inversely  proportional  to  these  numbers. 
Mixtures  of  gases  ought  to  have  a  mean  rate  of  their  constituent  gases ; 
from  which  rule,  however,  some,  as  hydrogen  and  carburetted  hydrogen, 
have  been  found  to  make  a  remarkable  exception,  their  rate  being  under 
such  circumstances  diminished  considerably  beyond  what  it  should  be. 

88 


PNEUMATICS.  89 

Thus,  only  1£  per  cent,  of  air  or  of  oxygen,  added  to  hydrogen,  was  found 
by  Graham  to  retard  its  passage  very  perceptibly,  and  at  least  3  times  more 
than  it  ought,  by  calculation. 

130.  If,  however,  instead  of  a  capillary  orifice  in  a  thin  plate,  a  capil- 
lary tube  of  the  same  diameter  be  substituted,  a  very  great  change  takes 
place  in  the  above  rates,  the  velocities  decreasing  rapidly,  as  the  orifice  is 
elongated  into  a  tube,  with  the  first  additions,  but  becoming  gradually  less 
affected,  and  after  a  certain  length,  they  remain  constant  for  any  further 
increase  in  the  length  of  the  tube.  By  a  comparison  of  these  ultimate 
velocities  for  different  gases,  it  is  found  that  the  ratios  between  them  re- 
main the  same  for  a  considerable  range  of  pressures  (from  1  to  y^th  Atm.), 
but  that  these  ratios  are  very  different  from  those  between  their  velocities 
through  capillary  orifices.  In  some  cases  they  approach  to  the  ratios  of 
their  different  densities,  but  not  uniformly  so.  Hydrogen  and  carburetted 
hydrogen  suffer  also  in  this  case,  by  admixture  of  other  gases,  a  considerable 
retardation  over  the  mean  of  their  mixture.  And  even  for  the  same  gas, 
the  velocity  is  found  to  change,  becoming  greater  as  the  density  of  the  gas 
is  increased,  so  that  the  higher  the  barometric  pressure  on  it,  in  the  less 
time  will  the  same  volume  of  gas  escape.  Graham  considers  this  a 
proof  that  the  effect  cannot  be  ascribed  to  friction,  and  he  therefore  dis- 
tinguishes the  flow  of  gases  into  a  vacuum  through  capillary  tubes,  from 
their  flow  into  the  same  through  capillary  orifices,  designating  the  latter 
by  the  name  of  E/usion,  while  their  flow  through  capillary  tubes  he  calls 
Transpiration.  When  the  space  into  which  the  gases  escape,  instead  of 
being  kept  vacuous,  is  allowed  to  become  filled  with  the  gas,  the  velocities 
decrease  slowly,  while  the  tension  of  the  gas  increases  from  1  to  10  inches, 
r  which,  however,  the  decrease  is  very  rapid  (Graham's  Chemistry,  p.  86). 

J.31.  When  gases  issue  under  pressure  into  the  Atmosphere,  they  seem 
also  to  obey  the  same  law  that,  for  different  gases,  their  relative  velocities- " 
under  the  same  pressure  are  inversely  as  the  square  roots  of  their  specific 
gravities.  For  the  same  gas,  its  velocities  under  different  pressures  are  as 
the  square  roots  of  these  pressures.  Thus,  according  to  Fyfe  (Edinb. 
New  Phil.  Journ.,  1848,  vol.  xlv.),  in  1  hour,  0.927  cub.  foot  of  common 
lighting  gas  (carburetted  hydrogen),  of  spec.  gr.  0.6026  (ref.  to  60°  as 
stand.),  will  pass  out  through  a  jet  formed  of  a  circular  orifice  of  -fa  inch 
in  diam.  under  a  pressure  of  j-JJ  inch  of  water  (burning  with  a  flame  5 
inch.  high).  Of  a  gas  of  0.500  sp.  gr.,  1.118  cub.  foot  will  pass  out  of 
the  same  jet  in  the  same  time  under  a  pressure  of  i-Jj  inch  of  water. 
\'  132.  When  a  gas  is  allowed  to  escape  under  pressure  from  an  orifice  in 
one  side  of  a  vessel,  no  pressure  can  of  course  be  exercised  by  the  gas 
on  this  orifice,  to  counteract  its  corresponding  pressure  on  an  equal  surface 

89 


90 


BOYE'S   INANIMATE   MATTER. 


Fig.  63. 


on  the  opposite  side  of  the  vessel,  hence  this  pressure  must  produce  a  ten- 
dency in  the  vessel  to  move  in  the  opposite  direction  of  that  in  which  the 
gas  flows  out.  This  may  be  seen  illustrated  in  the  revolving  gas-lights, 
seen  in  shop-windows  in  cities.  In  these  the  gas  is  made  to  enter  into 
two  lateral  branches,  see  a  and  a±  fig.  63,  which  are  capable  of  revolving, 
their  revolving  motion  being  produced  by  the 
gas  escaping  near  the  end  on  one  side,  while 
no  corresponding  orifice  or  jet  exists  on  the 
opposite  side,  as  seen  in  the  horizontal  section 
at  n  and  n±.  Instead  of  an  orifice  on  the  side 
of  the  lateral  branch  near  its  end,  the  same 
effect  is  produced  by  bending  sideways  the  end 
itself,  this  forming  the  jet. 

133.  If  a  thin  plate  of  metal  or  pasteboard, 
b  fig.  64,  be  perforated  at  its  middle,  and  fas- 
tened by  sealing-wax  or  otherwise  at  right 
angles  to  the  end  of  a  glass  tube  a,  so  that  the 
aperture  of  the  plate  is  directly  over  the  bore  of  the  tube,  and  another 
card  or  piece  of  stiff  paper  c  be  laid  over  the  opening,  having  a  pin  d 
stuck  through  it,  so  as  to  prevent  its  sliding  off,  it  will  be  impossible  to 
force  it  off  by  blowing  through  the  tube.  On  the  contrary,  if  the  apparatus 
be  inverted,  so  that  the  paper  is  lowermost,  blowing  through  the  tube  will 
prevent  it  from  falling  down,  and  the  greater  the  blast,  the  greater  will  be 
the  force  by  which  it  is  held  up.  This  experiment  is  called  the  Pneumatic 
Paradox.  The  cause  of  this  is,  that  as  the  air  from  the  tube  spreads  out 
when  escaping  between  the  plate  and  the  paper,  it  can  only  separate  them 
to  a  certain  distance  (about  ^  inch),  since  pushing  them  apart  beyond 
Fig.  64.  Fig.  65.  this,  would  cause  its  density  to  become  less  than 

that  of  the  atmospheric  air  on  the  other  side  of 
the  paper,  and  thus  produce  a  partial  vacuum 
between  them.  That  it  is  the  atmospheric 
pressure,  which  prevents  the  plate  and  the 
paper  from  being  separated,  can  be  proved  by 
furnishing  the  other  end  of  the  tube  with  a  screw 
s}  and  attaching  it  to  the  air-pump  plate,  placing 
over  it  a  receiver  with  a  stuffing-box  and  sliding 
rod,  by  which  the  paper  may  be  held  up  by  a 
loop  fastened  to  it,  till  the  air  is  exhausted,  and 
then  let  down  on  the  plate.  On  readmitting 
the  air  suddenly  through  the  tube,  the  paper  is  blown  off.  Fig.  65 

90 


PNEUMATICS. 


91 


exhibits  another  modification  of  this  experiment,  the  tube  terminating  in  a 
bowl  c.  By  blowing  through  the  tube  g,  a  ball  h  of  cork  or  any  other  light 
material,  will  remain  suspended,  instead  of  falling  or  being  blown  out. 

We  will  now  give  a  separate  consideration  to  the  class  of  gases  (45)  f,  \   . 
whicfilfce  called 


134.  Many  liquids  and  solids,  when  their  limit  is  towards  a  vacuum 
towards  a  gas,  are  capable  of  passing  wholly  or  in  part  into  the  gaseous 

form,  and  of  spreading  in  this  state  over  the  vacuum  or  through  the  gas. 
Such  liquids  and  solids  are  said  to  be  volatile,  while  those  which  are  not 
capable  of  assuming  the  gaseous  state  (as  oils),  or  owing  to  other  circum- 
stances, cannot  be  made  to  assume  it  (as  platinum),  are  said  to  be  fixed. 
The  gases  thus  formed  are  called  vapors.  This  conversion  into  vapors 
(vaporization)  may  take  place  either  only  from,  the  free  surface,  which  limits 
them  towards  the  vacuum  or  the  gas,  in  which  case  it  is  called  Evaporation, 
or  if  the  substance  be  a  liquid,  the  conversion  into  vapors  may  also  take 
place  below  the  free  surface,  the  vapors  escaping  as  bubbles  through  the 
liquid  and  agitating  it,  in  which  case  it  is  called  Ebullition  or  Boiling. 

135.  All  vapors,  being  true  gases,  are,  therefore,  perfectly  transparent; 
and,  when  colorless,  as  invisible  while  vapors,  as  all  other  gases,  until 
they  again  assume  the  liquid  or  solid  state,  and  at  that  moment  again  cease 
to  be  vapors.     It  is,  therefore,  a  popular  error  to  apply  the  word  steam, 
by  which  we  understand  vapor  of  water,  to  the  smoke  or  cloud  formed  by 
the  particles  of  liquid  water,  into  which  the  steam  again  condenses  at  a 
short  distance  from  a  steam-pipe,  when  escaping  into  the  atmosphere. 
Near  the  pipe,  where  it  is  yet  real  steam  or  vapor,  it  is  as  invisible  as  the 
rest  of  the  atmosphere.     Even  vapors  of  perfectly  opaque  bodies  are  trans-., 
parent  and  in  many  cases,  such  as  that  of  mercury,  also  perfectly  colorless. 
In  other  cases,  although  always  transparent,  they  may  possess  color.     Thus, 
vapor  of  sulphur  is  yellow,  and  vapor  of  Iodine  is  of  a  beautiful  violet  color. 

Formations  of  vapors  in  a  vacuum. 

136.  To   illustrate   the  formation  of  vapors  from  volatile  substances, 
when  limited  towards  a  vacuum,  we  may  employ  a  Torricellian  Tube  (60), 
inverted  in  a  large  cup  of  mercury,  see  1  fig.  66,  and  furnished  with  an 
accurate  scale  to  measure   the  height  of  the   mercurial  column.     This 
column,  which  is  supported  by  the  atmospheric  pressure,  we  will  sup- 
pose to  be  exactly  30  inches.     If  we  now  introduce,  through  the  mercury 
in  the  cup  d}  the  smallest  possible  quantity  of  water  into  the  tube  1,  it 

91 


92 


BOYE'S   INANIMATE  MATTER. 


will  rise  to  the  top  of  the  mercury  at  30,  and  thus  present  an  upper  or 
free  surface  towards  the  Torricellian  Vacuum.     It^will  then  in  a  short 
Fi9  66-  time  be  found  to  disappear  as 

water,  being  converted  into 
vapor,  the  presence  of  which 
as  a  gas  in  the  vacuum  is  in- 
dicated by  its  property  of  ex- 
pansibility, that  is,  its  spread- 
ing over  the  vacuum  with  a 
certain  force,  until  it  is  resisted 
by  the  limits  of  the  latter,  viz. 
the  sides  of  the  tube  and  the 
top  of  the  mercury  at  30, 
thus  causing  a  certain  uniform 
pressure  on  them  all,  called 
its  tension,  and  by  which  the 
mercury  becomes  slightly  de- 
pressed below  its  former  level 
at  30.  By  introducing  addi- 
tional small  quantities  of 
water,  we  shall  find  that  the 
same  continues,  the  water  dis- 
appearing as  liquid,  and  the 

mercury  becoming  more  depressed,  until  at  last  no  more  water  is  found  to 
disappear,  and  no  more  depression  occurs,  however  much  water  we  may 
introduce,  provided  the  temperature  remains  the  same.  Thus,  if  the 
experiment  be  performed,  when  the  stand  of  the  mercury  is  30  inches  and 
the  temperature  59°  Fah.,  this  depression  will  stop  at  £  inch  at  6,  see  tube 
1  fig.  66,  or  when  the  mercury  has  a  height  of  29  J  inches.  If  on  the 
contrary,  the  temperature  be  raised,  more  liquid  will  again  disappear,  more 
vapor  be  formed,  and  the  depression  of  the  mercury  become  greater,  till  at 
last,  when  it  has  reached  a  certain  point,  it  again  becomes  stationary.  If 
the  temperature  be  raised  to  79°,  this  will  occur  when  the  depression 
becomes  1  inch,  or  when  the  height  of  the  mercurial  column  is  29  inches,  after 
which  the  depression  does  not  increase  any  further,  as  long  as  the  tempe- 
rature remains  the  same;  and  so  on.  We  conclude  from  this,  that  the 
formation  of  vapors  from  volatile  liquids  in  a  vacuum  has  a  limit,  which 
depends  on  the  temperature,  so  that  for  every  temperature,  there  is  a  cer- 
tain greatest  or  maximum  quantity  of  vapor  which  can  be  taken  up,  with 
a  corresponding  maximum  tension,  beyond  which  no  more  can  be  taken  up. 

92 


PNEUMATICS. 


137.  An  otherwise  vacuous  space  may  therefore,  at  a  certain  tempera- 
ture, contain  less  than  this  maximum  quantity,  if  there  be  no  more  liquid 
present  to  form  more  vapor,  but  it  can  never  contain  more.  The  quantity 
which  is  present,  whether  it  be  the  maximum  or  less,  is  always,  for  the 
same  temperature,  proportional  to  its  tension,  or  the  pressure  which  it 
causes  on  the  mercury.  Should  the  temperature  not  be  the  same,  a 
deduction  must  first  be  made  from  its  tension  at  the  higher  temperature 
of  so  much,  as  is  due  to  the  expansion  of  the  vapor  by  heat  by  the  diffe- 
rence in  temperature  (see  140  and  100).  If,  on  the  other  hand,  we  can 
prove,  that  a  space  contains  the  maximum  quantity,  or,  as  it  is  termed,  is 
filled  to  saturation  with  vapor,  which  may  be  known,  for  instance,  by  its 
having  been  sufficiently  long  in  contact  with  an  abundance  of  the  liquid, 
then  we  may,  from  the  temperature,  estimate  the  quantity  of  vapor  in  the 
space,  and  its  tension,  since  these  will  be  the  maximum  quantity  and  tension, 
which  correspond  to  the  temperature. 

138.  By  experiments,  the  following  temperatures  have  been  found  to 
correspond  to  the  annexed  maximum  tensions  and  quantities  of  vapor  of 
water : 


Temp. 
Fahren. 


Max.  tens. 

in 
inch,  of  mercury. 


Max.  quan. 

in  1  cub.  foot, 

in  grains. 


Temp. 
Fahren. 

Max.  tens, 
in  Atmos. 

Max  quan. 
in  1  cub.  foot, 
in  grains. 

149°.65 

i  Atmos. 

70.640 

179°.08 

4        " 

134.766 

9KK  '-t  IT 

250°.52 

2        « 

ZuO.Oil 

484.791 

293°.72 

4       " 

913.951 

341°.78 

~X          s* 

8       " 

1718.225       X^1* 

-^«7^ 

le  it  will  be  seen,  that  at  212°  the  maximum  tensio 

of  the  vapor  of  water  is  equal  to  the  atmospheric  pressure,  and  that  it- 
therefore  at  that  temp,  will  cause  a  depression  of  the  mercury  inside  the 
Torricellian  tube  to  the  same  level  as  outside.  The  tension  o'r  elasticity 
for  higher  temperatures  than  212°  cannot,  therefore,  be  conveniently 
estimated  in  the  same  apparatus  as  described  above,  but  we  may  then  sub- 
stitute for  it  the  apparatus  represented  in  fig.  67,  consisting  of  a  small 
boiler  I  I,  furnished  with  a  mercurial  pressure-gauge  c  g,  (106),  the  cistern 
of  which,  c,  communicates  by  an  opening  with  the  vapor  inside  the  boiler, 
so  that  by  it  we  estimate  the  tension,  while  the  temperature  is  indicated 
by  the  thermometer  t.  The  boiler  is  also  furnished  with  a  stop-cock  i. 
The  boiler  having  been  partly  filled  with  water,  the  latter  is  made  to  boil 
by  the  application  of  heat.  As  soon  as  the  escaping  steam  has  expelled 
completely  the  atmospheric  air,  the  stop-cock  i  is  closed.  The  tension  of 

93 


94 


BOYE'S  INANIMATE  MATTER. 


the  vapor  will  then  be  found  to  increase  rapidly,  being  indicated  by  the 
Fi9-  6r-  height  of  the  mercurial  column  in  the  gauge, 

while  the  corresponding  temperatures  are  in- 
dicated by  the  thermometer.  In  the  experi- 
ments performed  for  the  French  Academy  in 
1829,  by  Arago  and  Dulong,  for  estimating 
the  elasticity  of  steam  at  higher  temperatures, 
the  highest  tension  measured  was  24  Atmo- 
spheres. The  tensions  were  estimated  by  a 
condensed  air-gauge  (107),  which  had  pre- 
viously been  tested  by  a  mercurial  gauge 
(106)  to  the  extent  of  27  Atmospheres. 
The  tube  of  this  latter  was  therefore  over  68 
feet  high,  having  been  ingeniously  constructed 
and  arranged  in  an  old  church-tower.  Mar- 
iotte's  law  was  thus  found  to  be  correct  to 
the  above  extent  (Annal.  de  fhim.  et  de 
Phys.,  2d  series,  vol.  xliii).  The  tensions 
below  212°  have  been  estimated  with  great 
accuracy  by  Regnault  (Ann.  de  Chim.  et  de 
Phys.,  3d  ser.,  vols.  xi,  xiv  and  xv). 

A  complete  set  of  tables  of  the  tensions  of 
vapor  of  water  in  English  inches,  and  the  temps,  in  Fahr.  degrees,  has 
been  computed  for  this  work  from  the  tables  furnished  by  these  authors, 
and  will  be  found  at  the  end  of  Pneumatics,  JJT1  TaMflfl  VTT.  VIII 

is  evident  from  these  and  the  above-given  table  (138)  of  the 
maximum  tensions  and  quantities  of  vapor,  that  these  increase  with  extra- 
ordinary rapidity  and  in  a  much  greater  ratio  than  the  temperatures,  when 
in  contact  with  the  liquid.  This  is  due  to  the  additional  vapors  formed 
from  it.*  •  If,  on  the  contrary,  at  any  time,  there  be  no  liquid  present,  the 
increase  in  tension  will  only  be  that  which  follows  from  the  expansion  of 
the  vapor  by  heat,  which  is  the  same  as  that  of  any  other  gas  under  the 

*  As  regards  the  tensions  of  vapors  at  very  high  temperatures,  it  would  seem  from  some 
interesting  experiments  of  Cagniard  de  la  Tour,  that  they  do  not  continue  to  increase  in  the 
same  augmented  ratio.  By  enclosing  volatile  liquids  in  sealed  glass  tubes,  and  exposing 
these  to  heat,  he  found  that  ether  passed  at  320°  entirely  into  the  state  of  vapor  in  a  space 
scarcely  double  its  own  volume,  and  without  exerting  a  pressure  of  more  than  38  Atmos. 
Alcohol  passed  into  the  gaseous  state  at  404i°,  in  a  space  of  3  times  its  own  volume, 
thereby  exercising  a  pressure  of  only  139  Atmos.,  and  water  (to  which  a  small  quantity  of 
Carbonate  of  Soda  had  been  added  to  prevent  the  breaking  of  the  tube),  in  a  space  4  times 
its  own  volume,  at  about  648°. 

94 


PNEUMATICS. 


95 


same  circumstances,  or  for  every  degree  Fahrenheit  0.00203611  of  its 
volume  at  32°,  or  0.00217802  of  its  volume  at  0°. 

141.  Conversely,  if  vapors  do  not  fill  the  space  to  saturation,  as  in  the  last- 
mentioned  case,  when  heated  to  a  higher  temperature  without  contact  with 
the  liquid,  or  when  allowed  to  spread  through  a  vacuum  in  a  less  quantity 
than  to  fill  it  to  saturation  at  the  existing  temperature,  such  vapor  may 
again,  without  becoming  liquid,  be  subjected  to  so  much  pressure  or  cold, 
as  will  again  reduce  it  to  the  state  of  saturation.     But  as  soon  as  the  pres- 
sure becomes  greater  than  its  maximum  tension  at  the  existing  temperature, 
it  will  all  be  reconverted  into  liquid;  and  if  the  temperature  becomes  less 
than  that,  at  which  its  tension  is  the  maximum,  a  portion  of  it  will  condense. 

142.  Thus,  as  an  illustration  of  this  in  Tegard  to  pressure,  suppose  that 
at  the  temperature  of  79°. 3  and  30  inches  barometric  stand,  the  Torricel- 
lian vacuum  b  a  c  fig.  68  tube  1,  contains  vapor  of  only  £  inch  tension, 

Fig  68.  that  is  only  £  the  maximum 

tension  and  quantity,  which 
belong  to  that  temperature. 
The  level  of  the  mercury  will 
then  of  course  be  at  29  £ 
inches,  or  at  b.  The  vapor 
being  thus  only  J  the  quantity 
that  can  exist  in  the  space,  it 
may  be  subjected  to  an  addi- 
tional pressure  of  J  inch,  or 
till  its  volume  is  compressed 
to  $  of  its  former  volume,  or 
into  c  a,  without  any  conden- 
sation taking  place.  This  in- 
crease in  pressure  is  produced 
by  inclining  the  tube,  as  tube  2 
in  the  fig.,  which  has  the 
effect  of  diminishing  the  Tor- 
ricellian vacuum  above  the 
mercury,  by  which  the  vapor 
becomes  more  compressed,  and 

its  density  and  tension  thereby  greater,  so  that  it  depresses  the  mercury 
more,  say  to  29 1  inch  at  bt.  The  compression  of  the  vapor  may  thus  be  in- 
creased by  still  farther  inclining  the  tube,  without  any  condensation  occur- 
ring, until  the  depression  in  the  perpendicular  height  of  the  mercury  is  1 
inch,  or  the  perpendicular  height  of  the  mercurial  column  29  inches,  see 

95 


96  BOYE'S  INANIMATE   MATTER. 

tube  3,  when,  in  consequence,  the  atmospheric  pressure  on  the  vapor  will  be 
30  —  29  inches,  =  1  inch  of  mercury.  At  the  same  time  the  vapor  will 
also  be  compressed  to  the  volume  ca  aa,  that  is  J  its  former  volume,  and  its 
tension  in  consequence  doubled  or  equal  to  1  inch.  The  pressure  on  the 
vapor  being  thus  equal  to  its  maximum  tension  at  that  temperature,  any 
farther  inclination  of  the  tube  will  not  cause  the  mercury  to  become  more 
depressed,  but  merely  diminish  the  Torricellian  space,  by  which  as  the 
space  become  diminished,  the  vapor  in  it  will  be  compressed  to  liquid  water, 
till  at  last,  when  the  top  of  the  tube  reaches  the  level  of  29  inches,  see  tube 
4,  no  vapor  will  remain,  all  having  been  converted  into^  liquid,  which  will 
appear  as  adrop  at  the  very  top  of  the  tube.-~""*"**-:p  >5i^2xV/\ 
"*T43.  In  tn"e  same  manner,  as  regards  temperature,  if  the  tube"  or  any 
other  vessel  containing  vapor,  not  filling  it  to  saturation,  be  subjected  to 
cold,  the  temperature  may  be  lowered  without  any  condensation  taking 
place,  until  it  reaches  that  degree  at  which  the  vapor  forms  a  maximum, 
after  which  a  portion  of  it  will  be  reconverted  into  liquid,  only  leaving  so 
much  vapor,  as  will  be  the  maximum  at  the  temperature  to  which  it  is 
cooled.  Thus,  as  in  the  above  case,  if  the  temperature  be  79°. 3,  and  the 
tube  contain  vapors  of  only  J  inch  tension,  which  is  only  J  the  maximum 
tension  and  quantity  corresponding  to  this  temperature,  it  may  be  cooled 
without  any  condensation  taking  place,  to  the  temperature  of  59°,  this 
being  the  temperature  at  which  its  tension  will  be  the  maximum.  But  if 
then  the  temperature  be  still  farther  lowered  to  40°,  so  much  of  it  will 
condense,  that  what  remains  has  only  a  tension  of  \  inch,  which  is  the 
maximum  at  that  temperature.  As  the  condensation  of  a  portion  of  the 
vapor  gives  the  appearance  of  a  dew  on  the  sides  of  the  vessel,  the  tempe- 
rature at  which  this  begins  to  take  place,  is  called  the  Dew  Point.  The 
condensation  of  a  portion  of  the  vapor  or  its  appearance  as  a  dew,  by  the 
slightest  increase  in  cold  or  pressure,  is  the  surest  proof  that  the  space  is 
filled  with  vapor  to  a  maximum  or  to  saturation. 

144.  The  formation  of  vapors  by  boiling,  will  take  place  whenever  the 
temperature  of  the  liquid  becomes  so  high,  that  the  maximum  tension, 
which  corresponds  to  its  temperature,  is  equal  to,  or  greater  than,  the  ten- 
sion or  pressure  of  the  vapor  on  its  free  surface.  By  this  the  liquid 
will  be  capable  of  forming  vapors  below  the  free  surface,  which  vapors 
generally  appear  as  small  bubbles  on  the  surface  of  the  containing 
vessel,  where  the  liquid  *is  in  contact  with  it,  and  which  bubbles  force 
their  way  through  the  liquid,  and  agitate  it.  In  a  close  vessel,  like 
that  of  fig.  67,  the  temperature  of  the  water  may,  therefore,  by  a  very 
gradual  heating  be  raised,  without  producing  boiling,  to  any  degree,  the 
maximum  tension  of  which  the  vessel  will  bear  without  bursting,  since 

96 


PNEUMATICS. 


97 


by  such  gradual  heating  the  formation  of  vapor  by  evaporation  from  the 
free  surface,  will  keep  pace  with  the  maximum  tension,  which  corresponds 
to  the  temperature  of  the  liquid.  If,  however,  the  vessel  be  heated  very 
suddenly  from  below,  so  as  to  raise  the  temperature  very  rapidly,  boiling 
may  be  produced  for  a  short  time,  till  the  tension  of  the  vapor  above 
becomes  the  maximum  for  the  temperature  of  the  liquid.  Another  much 
easier  way  of  producing  boiling  on  the  same  principle,  is  by  suddenly 
diminishing  the  tension  of  the  vapor  on  the  free  surface.  This  may  be 
done,  where  the  tension  is  greater  than  the  atmospheric  pressure,  as  in  the 
apparatus^.  67,  by  letting  the  vapors  escape  into  the  air  by  opening  the 
stop-cock  ij  by  which  a  violent  ebullition  will  take  place,  until  the  temp, 
of  the  liquid  is  again  lowered  to  212°,  which  is  the  temperature  which  cor- 
responds to  the  diminished  pressure  of  the  vapor  on  its  surface  (1  Atm). 
145.  Another  mode  of  diminishing  the  tension  of  the  vapors,  particularly 
if  less  than  the  atmospheric  pressure,  is  by  their  condensation,  absorption, 
or  exhaustion.  Thus,  the  production  of  boiling  by  condensation  of  the 
vapors,  by  applying  cold  to  that  portion  of  the  vessel  where  they  are  con- 
Fig.  69.  tained,  may  be  illustrated  by  an  experiment,  known 
under  the  name  of  the  Culinary  Paradox  (so  called 
because  it  produces  boiling  by  cold),  which  consists  in 
boiling  water  in  a  globular  glass  vessel  with  a  long 
neck  (bolt-head),  till  all  the  atmospheric  air  is  expelled. 
It  is  then  quickly  closed  up  by  a  cork,  while  removing 
it  from  the  fire,  and  inverted  as  in  fig.  69.  By  apply- 
ing carefully,  so  as  to  prevent  its  breaking,  a  piece  of 
ice  or  a  sponge  moistened  with  cold  water  to  the  top  at 
c,  where  the  vapors  are  contained,  these  are  condensed, 
and  the  water  will  then  begin  to  boil  violently. 

146.  Strong  vessels  for  heating   liquids  to  a  high 
temperature,  furnished  with  a  safety-valve  to  regulate 
the  highest  temperature  of  the  liquid,  and  consequent 
pressure  of  the  vapor,  affording  the  latter  an  escape,  if  exceeding  a  certain 
Fig.  70.  limit,  are  known  under  the  name  of  Papin's  Digestor,  see 

fig.  70.  Such  have  been  applied  to  different  purposes  by 
the  greater  solvent  power,  acquired  by  liquids  at  tempera- 
tures higher  than  their  boiling  point  in  open  air;  for 
instance  for  the  extraction  of  gelatine  from  bones  by  water, 
or  the  solution  of  resinous  substances  for  varnishes  by  alco- 
hol or  oil  of  turpentine. 

147.  From  the  table  given  in  138  it  will  be  seen,  that 
G  97  9 


98  BOYE'S   INANIMATE   MATTER. 

water  continues  to  emit  vapors  many  degrees  below  its  freezing  point,  and 
that,  therefore,  even  ice  is  volatile.  The  question  therefore  arises :  do 
volatile  substances  continue  to  emit  vapors  at  all  temperatures,  however 
low,  although  of  course  in  a  continually  diminishing  ratio,  so  that  for  those 
substances  which  are  volatile  to  a  perceptible  degree  only  at  higher  tem- 
peratures, their  evaporation  becomes  at  last  inappreciable,  and,  therefore, 
imperceptible  at  lower  temperatures  ?  or  do  they  exhibit  at  a  certain  tem- 
perature a  theoretical  or  absolute  stop  to  the  further  formation  of  vapors  ? 
According  to  the  experiments  of  Faraday,  mercury  has  been  found  to 
begin  to  emit  a  very  small  but  perceptible  quantity  of  vapor  in  summer 
between  60°  and  80°;  but  in  winter  the  formation  of  not  even  a  trace 
could  be  detected  by  the  most  delicate  tests.  It  seems  therefore  probable, 
that  volatile  substances  cease  all  at  once  to  emit  vapors,  and  that  this  point 
will  be  arrived  at,  when  their  expansive  or  evaporative  power  becomes 
so  small,  that  it  is  counteracted  or  overcome  by  the  forces  of  cohesion  and 
^gravity  (compare  also  27). 

*  i4#T  The  maximum  quantities  and  corresponding  maximum  tensions 
of  other  volatile  substances  for  the  same  temperatures,  are  different  from 
those  of  water,  being  greater  for  the  same  temperature,  the  more  volatile  they 
are.  An  idea  of  their  relative  volatility  may  be  obtained  by  referring  to 
their  boiling-point  in  air  (see  154),  which  indicates  the  temperature  at  which 
their  maximum  tension  is  the  same  as  that  of  water  at  212°.  The  lower 
their  boiling  point,  of  course  the  greater  is  their  volatility.  But  the  ratio 
of  the  increase  of  the  tension  of  their  vapor,  to  the  increase  in  the  tempera- 
ture, is  somewhat  different  for  the  different  substances.  Thus  the  boiling- 
point  of  mercury  is  662°,  and  the  tension  of  its  vapor  at  that  temperature, 
therefore,  30  inches,  or  1  Atm.  For  lower  temperatures  Regnault  obtained 
the  following  maximum  tensions  of  its  vapor  in  a  vacuum : 

Temperature      212°.2         144°.93  120°.47  77°.7 

Tension  0.160  in.     0.0072  in.         0.0034  in.        0.0013  in. 

149.  It  will  be  evident  from  the  foregoing,  that  the  conversion  of  vola- 
tile substances  into  vapors  in  a  vacuum  is  facilitated :  1st,  by  an  increase 
in  the  temperature,  and,  2d,  by  the  removal  of  the  vapor  as  fast  as  it 
is  formed.     The  latter  may  be  effected  either  by  condensation,  by  the 
external  application  of  cold  to  a  different  part  of  the  vacuum  at  a  distance 
from  the  liquid;    by  absorption,   by  placing  in  a  different  part  of  the 
vacuum  a  substance,  that  by  its  adhesion  or  chemical  affinity  will  attract 
and  thereby  remove  the  vapors ;  or  in  some  cases  by  exhaustion  of  the 
vapor  by  an  air-pump. 

150.  In  the  same  manner  as  the  removal  of  the  atmospheric  pressure 

98 


PNEUMATICS. 


99 


will  cause  the  expansibility  of  gases  to  overcome  their  adhesion  to  solids 
(121)  or  liquids  (120),  so  the  placing  of  volatile  liquids  in  a  vacuum  will 
have  the  same  effect,  causing  their  expansive  or  evaporative  power  to  over- 
come their  adhesion  or  even  feeble  chemical  affinities.  Hence  in  chemistry, 
desiccation  or  drying,  evaporation  and  boiling,  and  the  expulsion  of  che- 
mically combined  water,  are  often  effected  or  assisted  by  placing  such  sub- 
stances with  suitable  arrangements  in  a  vacuum. 

^      ^ 

S\ 


rmation  of  vapors  in  a 

151.  To  illustrate   the  formation  of  vapors  from  liquids,  when  their 
limit  is   towards  a  gas,   we   may  use    several   receivers,    see   d  and   Ti 
fig.  71,  filled  with  different  gases,  such  as  atmospheric  air  and  hydrogen, 
Fig.  71.  and   placed   in   a   pneumatic  cistern   </,  con- 

taining mercury,  one  side  of  which  should 
be  of  glass,  so  to  enable  us  to  observe  the  mer- 
^^H  curial  levels  inside.  The  receivers  having 
been  adjusted  so  that  the  mercury  has  the 
same  level  outside  and  inside,  the  gases  exer- 
cise themselves  the  same  tension  on  the  mer- 
cury, and  are  of  course  under  the  same  pres- 


sure,  as  the  air  outside,  that  isVojte  Atmo- 
sphere. If  we  now  introduce  into  those  two 
receivers,  as  before  into  the  Torricellian  vacuum 
(136),  a  small  quantity  of  water,  we  shall  find  that  it  in  the  same  man- 
ner disappears  as  liquid,  being  converted  into  vapor,  spreading  through  the 
gas  as  such,  and  indicating  its  presence  there  by  its  tension,  which  causes 
the  mercury  to  be  depressed  lower  inside  than  outside,  and  thus,  like  any^ 
other  gas,  adding  its  volume  and  tension  to  the  gas  to  which  it  mixed.  By 
introducing  additional  quantities  of  water,  the  same  will  be  repeated  until 
the  depression  of  the  mercury  has  reached  a  certain  point,  when  it  will 
increase  no  more,  the  water  remaining  liquid  and  no  more  vapor  being 
formed,  however  much  water  be  introduced,  provided  the  temperature  remain 
the  same.  If,  however,  this  be  raised,  more  vapor  will  be  formed,  and  the 
depression  increased,  till  it  again  becomes  stationary.  This  proves  that  the 
formation  of  vapor  in  a  gas  from  a  liquid,  has  a  limit  as  in  a  vacuum,  beyond 
which  no  more  can  be  taken  up,  so  that  for  a  certain  temperature  there 
may  be  less,  but  there  cannot  be  more,  than  this  maximum  quantity  with  a 
corresponding  maximum  tension.  As  the  vapor  in  every  case  adds  its 
tension  to  that  of  the  gas,  its  quantity  must  therefore  always  be,  making 
allowance  for  differences  in  temperature,  proportional  to  the  additional 

99 


100  BOYE'S  INANIMATE  MATTER. 

tension  acquired  by  the  gas.  What,  however,  is  very  extraordinary  is,  that 
the  maximum  quantities  for  the  same  temperatures,  which  can  exist  in  the 
different  gases,  are  the  same  for  them  ally  and  exactly  the  same  as  in  a 
vacuum. 

152.  Dr.  Dalton  of  England,  who  first  discovered  this  law,  connecting  it 
with  the  fact  that  gases  are  not  capable  of  resisting  each  other's  expansi- 
bility, or  of  limiting  each  other  (52),  expressed  it  in  this  manner,  that 
gases  are   to  each  other   as  vacua.     There   is,  however,  this  difference 
between  the  formation  of  vapor  in  a  vacuum  and  in  gases,  that  while  in  a 
vacuum  it  takes  place  very  rapidly,  and  the  maximum  quantity  is  attained 
soon,  it  is  much  slower  in  gases,  requiring  much  longer  time  to  attain 
the  maximum,  and  the  times  varying  for  different  gases,  being  shorter  for 
those   the   specific  gravities  of  which  are  less.     The  relative   times  for 
obtaining  the  maximum  Quantity  of  vapor  in  different  gases,  have  been 
found,  under  otherwise  similar  circumstances,  to  be  inversely  proportional 
to  the  square  roots  of  their  specific  gravities,  which  is  the  same  law  as  for 
diffusion.     This  seems  to  indicate  that  the  formation  of  vapor  in  a  gas 
depends  on  the  same  cause  as  the  penetration  of  gases  through  each  other 
by  diffusion,  and  therefore  depends  not  only  on  their  own  expansibility, 
but  also  on  the  attraction  of  the  atoms  of  the  gases  toward  each  other,  or 
adhesion.     Regnault  has  also  found  that  the  tension  of  vapor  of  water 
in  atmospheric  air  is  two  or  three  per  cent,  less  than  in  a  vacuum  at 
the  same  temperature,  and  that  its  density  also  has  a  slight  deviation, 
but  it  is  uncertain  whether  this  apparent  deviation  may  not  be  ascribed 
to  other  causes. 

153.  Boiling  depends  here,  as  in  a  vacuum,  on  the  same  principle,  and 
will  occur  whenever  the  maximum  tension  corresponding  to  the  tempera- 
ture of  the  liquid  is  greater  than  that  of  the  pressure  of  the  gas  and  the 
vapor  on  its  surface.     It  will  thus  be  seen  that  the  boiling  of  water  in  the 
atmosphere  must  occur  at  212°,  since  at  this  temperature  the  maximum 
tension  is  equal  to  the  pressure  of  the  atmospheric  air  on  its  surface,  and  its 
vapors,  therefore,  are  capable  of  sustaining  themselves  against  this  pressure, 
so  that  by  forcing  their  way  as  bubbles  through  the  water,  they  cause  the 
agitation,  which  we  call  boiling  in  open  air.     The  singing  or  hissing  noise, 
generally  called  simmering,  which  is  heard  just  before  boiling,  is  caused 
by  the  water  above  not  having  yet  acquired  the  full  temperature  of  212°, 
by  which  the  vapors  formed  at  this  temperature  below,  in  contact  with  the 
vessel,  are  again  condensed  by  contact  with  the  water. 

154.  The  more  volatile  substances  are,  the  greater  is  the  tension  or 
elastic  force  of  their  vapor  at  the  same  temperature,  and  the  lower  is  there- 
fore their  boiling-point  in  open  air.     The  following  table  exhibits  the  boil- 

100 


PNEUMATICS.  101 

ing-point  in  open  air  of  different  substances  at  the  mean  barometric  pres- 

sure of  the  atmosphere  of  29.918  inches  : 

Boiling-Point. 
Chlorohydric  or  Muriatic  Ether          ....  52° 

Ether  (a  liquid,  frequently  called  Sulphuric  Ether)      .  96° 

Alcohol  (Sp.  Gr.  0.798)  .....         173° 

Water  ........         212° 

Oil  of  Turpentine  ......         314° 

Oil  of  Vitriol  (Sp.  Gr.  1.845)    .....         620° 

Mercury          .         .         .         ./->   .         .         .         .       ,662° 


. 

155.  If,  however,  the  atmospheric  pressure  on  the  surface  of  the  water 
or  other  volatile  liquids  be  increased,  it  will  require  a  higher  temperature 
to  produce  boiling  ;  and  if  it,  on  the  contrary,  be  decreased,  boiling  will 
take  place  at  a  lower  temperature.     If,  therefore,  water  of  less  tempera- 
ture than  212°,  or  even  of  ordinary  high  temperatures  (70°  to  80°)  be 
placed  under  a  receiver,  and  the  air  quickly  exhausted,  it  will  begin  to  boil. 

156.  As  water  emits  vapors  of  a  certain  tension  at  all  temperatures,  it 
might  be  supposed  that  by  removing  all  pressure  from  its  surface,  it  could 
be  made  to  boil  at  any  temperature.     This  is,  however,  not  the  case,  as  it 
cannot  be  made  to  boil,  even  in  a  perfect  vacuum,  below  the  temperature 
of  67°.     The  reason  of  this  is,  that  although  at  this  temperature  it  is  yet 
capable  of  furnishing  vapor  of  a  tension  of  more  than  J  inch  of  mercury, 
this  tension  is  not  sufficient  to  overcome  the  pressure  caused  by  the  weight 
of  the  layer  of  liquid  above  it,  or  to  break  the  cohesion  of  its  particles. 
Other  volatile  liquids  have  a  similar  limit  or  lowest  temperature,  below 
which  they  cannot  be  made  to  boil  in  a  vacuum,  being  approximately  the 
same  number,  or  145°  below  their  boiling-point  in  open  air. 

157.  The  principle,  that  the  temperature  at  which  .  pure  water  boils 
depends  on,  and  varies  with  the  atmospheric  pressure,  being  always  that 
at  which  the  maximum  tension  of  its  vapor  is  equal  to  the  atmospheric 
pressure  on  its  surface,  is  used  in  the  construction  of  the  Boiling-Point 
Barometer,  described  in  87. 

158.  From  the  foregoing  it  will  be  evident,  that  the  conversion  of  vola- 
tile liquids  into  vapors  in  a  gas  in  a  close  vessel,  or  in  the  open  atmo- 
spheric air,  is  facilitated  :  1st,  by  heat,  and,  2d,  by  the  removal  of  the 
vapor  as  fast  as  it  is  formed.     This  latter  may  be  effected  by  condensation, 
by  applying  externally  cold  to  another  part  of  the  close  vessel  at  a  distance 
from  the  liquid  (Distillation)  ;  by  absorption,  by  placing  in  a  different  part 
of  the  close  vessel  substances,  which,  by  their  adhesion  or  chemical  affinity, 
will  attract  and  thus  remove  the  vapor  ;  or  by  displacement  of  the  satu- 

101 


102  BOYE'S   INANIMATE   MATTER. 

rated  air  over  the  liquid  by  less  saturated  or  perfectly  dry,  and,  in  some 
cases,  even  heated  air. 

159.  These  principles  are  often  applied  in  chemistry  for  effecting  or 
accelerating  the  drying  of  vessels  or  substances  containing  water.     Thus, 
the  drying  of  narrow-mouthed  vessels,  such  as  bottles,  which  even  by  heat- 
ing requires  considerable  time,  is  effected  in  a  few  moments  by  removing 
the  saturated  air  by  suction  through  a  tube,  the  other  end  of  which  is  intro- 
duced to  the  bottom  of  the  vessel. 

Vapor  of  Water  in  the  Atmosphere. 

160.  The  atmosphere  always  contains  Yapors  of  Water  (26),  which  are 
formed  by  evaporation  from  the  sea  and  the  moist  earth.     From  various 
causes  (92),  these  again  condense  to  liquid  water  either  on  the  surface  of 
the  earth  as  dew,  or  in  the  atmosphere  itself  as  small  hollow  spheres  or 
vesicles,  filled  with  air,  which  constitute  fogs  and  clouds. 

These  vesicles  may  be  observed  by  a  lens  of  1  inch,  focus  against  a  dark  ground.  Saus- 
sure  found  those  forming  the  mist  on  high  mountains  to  have  a  diameter  of  45^17  to  v-J-g-Q 
inch,  but  occasionally  to  be  as  large  as  a  pea.  A  fog  is  a  cloud  resting  on  the  earth.  On 
the  other  hand,  by  ascending  into  the  clouds,  these  appear  as  fogs.  According  to  Howard 
the  different  varieties  of  clouds  are  named  as  follows : 

Cirrus,  Curl-  or  Feather-Cloud,  composed  of  delicate  feathery  streaks  or  filaments,  more 
or  less  straight,  curly,  or  confused.  After  a  spell  of  fine  weather  they  are  generally  the 
first  to  change  the  blue  color  of  the  sky,  and  they  are  often  the  last  remaining,  when  the 
weather  becomes  fine.  They  are  the  highest  of  all  clouds,  and  have,  in  some  cases,  been 
estimated  to  have  an  elevation  of  20,000  feet. 

Cumulus,  Accumulated  or  Heap-Cloud,  forming  large  hemispherical  masses,  with  a  more 
or  less  horizontal  base.  They  are  often  piled  on  each  other,  and  when  lighted  by  the  sun, 
appear  as  mountains  of  snow.  In  hot  weather  they  frequently  appear  as  the  heat  of  the 
day  increases,  and  disappear  again  toward  evening. 

Cirro-Cumulus,  is  the  name  given  to  those  small,  white,  generally  rounded  clouds, 
arranged  in  rows,  mostly  with  the  blue  sky  visible  between  them.  After  rainy  weather, 
the  clouds  often  break  into  these,  and  they  give  to  the  sky  a  mottled  appearance  (Mackerel- 
back  sky). 

Stratus,  Layer-Cloud,  forms  a  misty  layer  of  clouds  near  the  earth.  It  often  forms  at 
sunset,  and  again  disappears  after  sunrise.  It  sometimes  resolves  itself  into  a  heavy  dew, 
at  other  times  it  rises  as  cumulus. 

Cirro-Stratus,  forms  streaks  or  bands,  but  heavier  than  the  cirrus,  which  often  passes 
into  it.  When  in  the  horizon  it  causes  the  beautiful  colors  of  the  sunset;  but  when  heavy 
gives  it  the  dark-red  appearance,  which  by  many  is  considered  as  the  precursor  of  rain. 
When  high  up,  it  often  appears  as  attenuated  clouds,  covering  the  sky  as  with  a  veil,  but 
at  other  times  it  assumes  a  darker  and  more  threatening  aspect. 

Cumulo-Stratus,  consists  of  dense  masses  and  layers.  It  is  generally  formed  by  the 
increase  of  the  cumulus,  extending  irregularly  at  the  top,  and  losing  its  straight  base  by 
the  addition  of  irregular  appendages  hanging  down  from  it.  It  is  then  apt  to  pass  into 
the  next. 

Nimbus,  or  real  rain-cloud,  characterized  by  its  uniform  grey  or  dark  appearance,  with 

102 


PNEUMATICS.  103 

fringed  or  indistinct  edges,  not  allowing  the  different  clouds  of  which  it  is  composed  to  be 
well  distinguished. 

The  word  Scud,  is  often  applied  to  the  loose  and  low  masses  of  clouds,  which  during  a 
storm  are  seen  to  move  with  great  rapidity  below  the  other  clouds,  and  often  in  a  different 
direction  from  them. 

When  the  vesicles  of  the  clouds  break  and  unite  into  solid  drops,  they  form  rain.  As, 
in  the  rule,  the  atmosphere  near  the  earth  must  always  become  saturated  with  vapors, 
before  rain  can  fall,  the  rain-drops  increase  in  their  descent  by  the  condensation  of  addi- 
tional vapors  on  their  surface,  and  their  size  therefore  depends  on  the  height  of  the  clouds. 
This  increase  is  very  perceptible  by  measuring  the  quantity  of  rain  falling  at  different 
heights  in  the  same  place.  Thus,  an  increase  in  the  annual  amount  of  rain  of  over  one-half, 
has  been  observed  in  a  fall  of  240  feet.  The  amount  of  rain  which  falls  is  estimated  in 
inches,  indicating  the  depth  of  the  layer  of  water  which  it  would  form,  if  allowed  to 
remain  standing  on  the  earth.  The  instrument  used  for  this  purpose  is  called  the  Rain- 
gauge  or  Ombrometer,  and  consists  of  a  funnel,  the  mouth  of  which  has  a  known  area,  and 
which  discharges  the  water  into  a  large  bottle  or  other  suitable  vessel  of  sufficient  capacity, 
in  or  from  which  it  is  measured  in  cubic  inches.  The  number  of  cubic  inches,  divided  by 
the  number  of  square  inches  constituting  the  area  of  the  mouth  of  the  funnel,  gives  the 
height  or  depth  of  the  water  fallen.  Thus,  if  the  mouth  of  the  gauge  be  circular 
and  7.98  inch,  in  diameter,  each  cubic  inch  of  water  will  correspond  to  0.02  inch  of  rain. 
Rain-gauges  may  also  be  made  self-registering  (Osier's).  The  annual  amount  of  rain 
increases  from  higher  latitudes  toward  the  equator,  varying  from  13  to  126  inches.  In 
Philadelphia  (Penn.  Hospital)  it  is  44  inches.  But  the  number  of  rainy  days,  over  which 
the  fall  of  the  rain  is  distributed,  varies  in  the  reverse  order. 

Hailstones  are  frozen  rain-drops,  their  size  increasing  by  a  prolonged  suspension  in  the 
atmosphere  by  powerful  upward  currents  or  by  electricity.  Snow  is  formed  by  the  freezing 
of  vapor  or  of  the  vesicles.  Snow-flakes  often  exhibit  the  most  beautiful  starlike  appear- 
ances, varying  much  in  the  form  of  their  rays,  but  are  always  of  the  same  form  in  the  same 
snow-fall.  Their'  form  is  produced  by  the  different  small  crystals  of  which  the  flake  is 
composed,  arranging  themselves  in  different  manners,  although  always  at  the  same  angles. 

161.  The  two  most  important  forms  in  which  water  exists  in  the  atmo- 
sphere,  are,  therefore,  in  the  liquid  state  as  vesicles,  and  in  the  gaseous 
state  as  vapor.  Both  states  constitute  what  is  commonly  (see  162)  under- 
stood by  the  dampness  or  moisture  of  the  atmosphere.  When,  however, 
the  atmosphere  is  perfectly  transparent,  the  water  may  be  considered  as- 
existing  entirely  in  the  state  of  vapor.  But  even  in  this  state,  when 
approaching  the  point  of  saturation,  it  imparts  to  the  atmosphere  ?  .f1  '^ided 
dampness;  and  by  depressing  the  perspiration  of  the  skin,  which  cannot 
pass  off  as  vapor,  when  the  air  is  saturated,  it  causes  such  air,  if  cold,  to 
feel  chilly  and  harsh  or  raw,  and,  when  hot,  sultry  and  oppressive.  In  the 
same  degree  also,  as  the  air  approaches  the  state  of  saturation,  the  tendency 
of  the  vapor  to  precipitate  in  the  liquid  state,  increases,  and  it  therefore 
becomes  important  to  estimate  at  any  time  the  vapor  in  the  atmosphere, 
ind  its  approach  to  saturatio 

or  humidity,  or  absolute  moisture  or  humidity, 
in  the  meteorological  sense,  is  understood  the  quantity  of  water,  which 
exists  in  the  atmosphere  in  the  state  of  vapor,  while  by  relative  mois- 

103 


104 


BOYE'S   INANIMATE   MATTER. 


Fig.  72. 


ture  or  humidity,  is  understood  the  fraction  which  this  constitutes  of  the 
maximum  quantity  or  of  saturation  for  the  existing  temperature.  Thus, 
a  relative  humidity  of  0.31  means,  that  the  atmosphere  contains  y^ths  of 
the  quantity  of  vapor,  which  at  the  temperature  in  question,  whatever  this 
may  be,  would  constitute  saturation.  Instead  of  referring  the  relative 
humidity  to  saturation  as  1,  it  is  often  referred  to  it  as  100,  in  which  case 
the  above  relative  humidity  will  be  31.  It  is  therefore  on  the  relative 
moisture,  and  not  on  the  absolute  quantity  of  vapor,  that  what  is  commonly 
called  the  dryness  of  the  air  depends,  for  if  the  quantity  of  vapor  only 
forms  a  small  portion  of  the  quantity  which  constitutes  saturation,  the  air 
will  yet  freely  take  up  more  vapor,  and  therefore  appear  dry.  Thus  the 
same  air  that  in  winter  is  called  damp,  will  in  summer,  when  the  tempera- 
ture is  higher,  appear  dry. 

163.  The  most  accurate  way  of  esti- 
mating the  quantity  of  vapor  in  the  atmo- 
sphere is  by  the  chemical  method,  see  fig. 
72,  which  consists  in  passing  a  known 
volume  of  air  though  a  U-shaped  tube  e, 
filled  with  pieces  of  pumice-stone,  pre- 
viously moistened  with  oil  of  vitriol, 
which  absorbs  all  the  vapor  from  it.  The 
air  is  drawn  very  gradually  through  this 
tube  by  connecting  it  with  the  aspirator  g 
filled  with  water,  which  latter  is  allowed  to 
run  out  very  slowly  through  the  stop-cock 
«,  and  thereby  draws  the  air  through  the 
tube  e}  to  replace  it.  The  tube  c  is  also 
filled  with  pumice,  moistened  with  oil  of 
vitriol,  but  is  permanently  attached  to 

the  aspirator,  to  prevent  any  vapor  passing  from  it  into  the  tube  c.  The 
tube  d  is  similarly  filled,  but  serves  only  as  a  check  to  ascertain  whether 
all  the  vapor  has  been  absorbed  by  the  tube  e,  and  may  be  dispensed 
with.  The  tube  e  is  weighed  accurately  before  and  after  the  experiment, 
and  its  increase  in  weight  is  the  amount  of  vapor  in  the  volume  of  air 
drawn  through  it  by  the  aspirator  #.  This  volume  is  estimated  by  measuring 
the  quantity  of  water  which  it  holds.  A  strict  account  must  be  kept  of 
the  temperature  of  the  air  during  the  experiment,  by  placing  a  thermometer 
at  /,  where  it  enters  the  tube.  The  aspirator  is  also  furnished  with  a 
thermometer  b  u}  and  should  its  temperature  at  the  end  of  the  experiment 
differ  from  the  average  temperature  of  the  air  which  entered,  its  volume 

104 


PNEUMATICS.  105 

must  be  reduced  to  the  same,  making  also  a  deduction  for  the  quantity  of 
vapor  in  it,  and  for  any  variation  in  the  barometric  pressure  during  the 
experiment  (100).  Should  the  state  of  moisture  of  the  room  in  which  the 
experiment  is  performed  be  different  from  that  of  the  atmosphere,  the  air 
must  be  drawn  in  from  the  outside  by  a  longer  tube.  Having  thus 
obtained  by  weight  the  absolute  quantity  of  vapor  in  a  certain  volume  of 
the  atmosphere,  the  relative  humidity  is  easily  obtained  by  dividing  this 
obtained  quantity  by  the  maximum  quantity  for  the  same  volume  (168), 
corresponding  to  the  observed  temperature  of  the  air;  or  the  tension  of  the 
vapor  may  be  calculated  from  the  obtained  weight  (168)  and  divided  by 
the  maximum  tension  for  the  temperature  of  the  air  (164).  This  method 
allows  us  also  to  estimate  the  quantity  of  vesicular  water  existing  in  the 
atmosphere,  since  in  such  case  the  air  must  be  saturated  with  vapor,  and 
its  quantity,  therefore,  equal  to  the  difference  between  the  quantity 
obtained  by  the  experiment,  and  the  maximum  quantity  for  the  tern] 
ture.  It  has,  however,  the  inconvenience,  that  it  requires  longerTime, 
considerable  skill  in  the  operator,  and  expensive  apparatus,  particularly  K 
for  weighing  the  tube  with  sufficient  accuracy.  Other  methods  and  instru-  V 
ments  have  therefore  been  contrived,  which  will  now  be  described.^ 

HYGROMETERS. 

164.  By  hygrometers  (from  hypos  (hugros)  moist,  and  fjLsrpov  (metroii), 
measure),  we  understand  instruments  for  estimating  the  moisture  of  the 
atmosphere.  The  best  of  these  act  on  the  principle  of  finding  the  Dew- 
Point,  that  is,  the  temperature  at  which  the  vapor  existing  in  the  atmo- 
sphere would  be  the  maximum  quantity  or  fill  it  to  saturation.  This  is 
done  by  cooling  a  portion  of  it  till  the  vapors  condense  as  a  dew  (143), 
and  then  observing  the  exact  temperature  at  which  this  begins  to  take 
place,  which  temperature  constitutes  the  dew-point.  As  the  vapor  in  the 
atmosphere  is  not  confined,  but  free  to  contract  or  expand,  the  maximum 
tension  corresponding  to  its  dew-point  must  be  the  same  as  its  tension  in 
the  atmosphere  at  the  existing  temperature,  and  will  therefore  bear  the  same 
ratio  to  the  maximum  tension  corresponding  to  the  temperature  of  the 
atmosphere,  as  its  quantity  bears  to  the  maximum  quantity  for  this  same 
temperature.  We  therefore  obtain  the  relative  humidity  of  the  atmosphere 
~by  dividing  the  maximum  tension,  corresponding  to  the  temperature  of  the 
Dew-Pointy  by  the  maximum  tension  corresponding  to  the  temperature  of 
the  atmosphere.  For  this  purpose  the  maximum  tension  for  every  0.2  degree 
Fah.  from  104°  to  0°  will  be  found  in  Table  IX,  at  the  end  of  Pneum. 

Thus,  suppose  that  the 

Dew-Point  ==  60°  Temp,  of  Aim.  =  85° ; 

105 


106  BOYE'S  INANIMATE   MATTER. 

we  then  have  from  Table  IX, 

Max.  Tension  for  60°  =  0.518  inch 
"  "       "   85°  =  1.203    « 

therefore :  Relative  Humidity  =  °-518  =  0.431 ; 

that  is,  the  atmosphere  contains  y^^ths  of  the  quantity  of  vapor,  which  it 
is  capable  of  taking  up,  and  which  would  constitute  saturation  at  its  tem- 
perature of  85°.  Instead  of  referring  to  saturation  as  1,  the  relative 
humidity  is  often  referred  to  it  as  100.  In  the  above  case  it  would  then 
be  43.1. 

165.  Conversely  to  find  from  the  relative  humidity  and  the  temp,  of  the 
atinos.,  the  tension  of  the  vapor  in  it  and  the  dew-point,  we  multiply  the 
max.  tens,  for  the  temp,  of  the  atmos.,  taken  from  Table  IX,  by  the  rel. 
humidity  referred  to  saturation  as  1,  which  gives  us  the  tension  of  the  vapor 
in  the  atmos.,  and  as  this  is  also  the  max.  tension  for  the  dew-point,  the 
temp,  which  in  Table  IX  corresponds  to  this  tension  is  the  dew-point,  f 

166.  To  obtain  the  quantity  of  vapor  in  the  atmosphere,   either  ly 
volume  or  l>y  weight,  referring  it  to  the  atmospheric  air  itself  as  1  (which 
if  referred  to  it  as  100,  constitutes  the  per  centage  by  volume  or  by  weight), 
we  may  consider  vapor  of  water  as  obeying  Mariotte's  law,  both  as  regards 
its  volume  and  its  density  in  the  atmosphere.     To  estimate,  therefore,  its 
volume,  it  must  be  kept  in  mind,  that  while  occupying  the  whole  volume 
of  the  atmosphere  through  which  it  is  diffused  (the  observed  volume),  it 
only  sustains  so  much  of  the  atmospheric  pressure  as  is  equal  to  its  own 
tension  /,  and  that  to  obtain  the  true  volume  V,  which  it  would  occupy 
under  the  whole  atmospheric  pressure  B  (see  100),  we  have  that : 

F:  Vol.  of  Atmos.  ::1  :1 

£    f 
therefore,  calling  the  volume  of  the  atmosphere  1,  we  have 

/  being  =  the  tension  of  the  vapor,  which  is  the  same  as  the  maximum 
tension  for  the  dew-point,  and  B  =  the  stand  of  the  Bar.  Thus,  suppose 
the  dew-point  =  60°,  and  the  stand  of  the  Bar.  =  29  inch.,  we  then  have 
from  Table  IX,  that  the  max.  tension  for  60°  =  0.518  inch,  and  therefore : 

V  =  0^15.=  0.01786; 
29 

that  is,  the  volume  of  the  vapor  constitutes  0.01786  of  that  of  the  atmo- 
sphere, or  it  is  1.786  per  cent,  by  volume. 

167.  To  obtain  the  weight  of  the  vapor  in  the  atmosphere,  referred  to 
that  of  the  atmosphere  itself  as  1,  we  multiply  the  tension  of  the  vapor  by 
0.622  (Sp.  grav.  of  Yap.  Water),  and  divide  this  product  by  itself  after 

106 


PNEUMATICS.  107 

having  added  to  it  the  difference  between  the  stand  of  the  Barometer  and 
the  tension  of  the  vapor.  Or,  calling  the  weight  of  the  vapor  W}  we  have : 

0.622  / 

=  (B—f)  -f  0.622  / 

/  being  =  the  tension  of  the  vapor,  which  is  the  same  as  the  maximum 
tension  for  the  dew-point,  and  B  =  the  stand  of  the  Barometer.  Thus, 
suppose,  as  in  the  former  case,  the  stand  of  the  Barometer  =  29  inch,  and 
the  dew-point  60°,  we  then  have  as  before,  from  Table  IX,  the  maximum 
tension  for  60°  =  0.518,  therefore: 

W °'622  *  0-518 =0.01119; 

(29  —  0.518)  +  0.622  X  0.518 

that  is,  the  weight  of  the  vapor  is  0.01119  of  that  of  the  atmosphere,  or  it 
is  1.119  per  cent,  by  weight. 

168.  To  obtain  the  absolute  weight  of  the  vapor  in  a  given  volume ,  for 
instance,  in  1  cubic  foot  of  the  atmosphere,  or  what  is  the  same,  since  this 
quantity  is  the  same  as  if  the  space  contained  no  air,  the  absolute  weight 
of  1  cubic  foot  of  Vapor  of  Water,  we  have  by  Mariotte's  law,  as  stated 
in  166,  that  the  densities  of  the  vapor  in  the  air  at  ordinary  temperatures 
may  be  considered  proportional  to  the  pressures  on  it,  the  pressure  on  it 
at  any  time  being  the  same  as  its  tension.  We  know  also  that  its  expan- 
sion by  heat  is  the  same  as  that  of  other  gases  (140).  To  find,  therefore, 
the  weight  of  vapor  in  1  cubic  foot  of  the  atmosphere,  or  1  cubic  foot  of 
vapor  of  the  tension  and  temperature  in  which  it  exists  in  the  atmosphere, 
we  proceed  as  directed  in  100,  by  first  reducing  1  cubic  foot  (considering 
this  as  the  observed  volume  of  vapor)  to  the  standard  pressure  (29.918 
inch.)  and  temperature  (32°),  and  then  multiply  the  thus-reduced  volume, 
first,  by  the  weight  of  1  cubic  foot  of  atmospheric  air  of  the  same  standard 
pressure  and  temperature  (=  563.1007  grains),  and  then  by  the  specific 
gravity  of  Vapor  of  Water  (=  0.622),  so  that  calling  the  weight  of  the 
vapor  in  1  cubic  foot  W,  we  have : 

W=  1  X  297918  X  1+0.0020861  (<-32°)  X  563-1007  ^  X  °'622 

=  11.7055  grs.  X  1  +  0.0020361  ($—82°)' 

f  being  =  the  tension  of  the  vapor,  which  is  the  same  as  the  maximum 
tension  for  the  dew-point,  and  t  =  the  temperature  of  the  atmosphere,  or 
of  the  vapor.  Thus,  suppose  the  dew-point  =  60°,  and  the  temperature 
=  85°,  we  then  have  from  Table  IX  the  maximum  tension  for  60°  =  0.518 
in.,  and  therefore  the  weight  of  vapor  in  1  cubic  foot,  W: 

0.518 

(F=  11.70660".  X    1+00620861  (85Q-32°) 

=  5.473  grains, 

107 


108 


BOYE'S   INANIMATE  MATTER. 


By  actual  experiments,  Regnault  found  that  the  quantities  thus  calculated 
on  the  above-stated  supposition,  that  vapor  of  water,  when  diffused  through 
air,  obeys  Mariotte's  law,  and  that  its  tensions  and  densities  are  the  same 
as  in  the  vacuum,  were  only  about  1  per  cent,  greater  than  those  obtained 
by  actual  weighing  of  the  vapor  (compare  152). 


Hygrometers  giving  the  Dew-Point. 

169.  DanieWs  Hygrometer. — It  consists  of  a  mode- 
rately wide  glass  tube,  see  ah  fig.  73,  blown  out  at  its 
two,  extremities  into  bulbs,  and  bent  twice  at  right 
angles.  One  bulb  is  partly  filled  with  liquid  ether, 
while  the  rest  of  the  apparatus  is  freed  from  atmo- 
spheric air,  but  contains,  of  course,  vapor  of  ether. 
The  bulb  d  containing  the  ether,  has  a  thermometer 
inside ;  while  the  other  bulb  c  is  covered  with  thin 
muslin.  To  use  it,  we  first  pour  ether,  drop  by  drop, 
on  the  bulb  c,  which  ether,  by  its  evaporation,  pro- 
duces cold  (see  Latent  Heat  under  Thermics),  and  thereby  condenses  the 
vapor  inside.  By  this,  the  tension  or  pressure  of  the  vapor  on  the  liquid 
ether  in  the  other  bulb  d  is  removed,  and  the  ether  in  it  thereby 
begins  to  boil,  or  evaporate  very  rapidly  (145).  The  temperature  of  the 
remaining  ether  in  the  bulb  is  thus  lowered,  and  thereby  that  of  the  bulb 
itself  and  the  atmosphere  surrounding  the  bulb  on  the  outside ;  till  at  last 
the  vapor  of  the  atmos.  forms  a  maximum,  and  then  begins  to  condense  on 
the  outside  of  the  bulb  as  a  dew.  At  this  moment  the  temperature  of  the 
bulb  is  observed  by  the  thermometer  inside,  and  this  gives  the  temperature 
of  the  dew-point  of  the  atmosphere.  As  this  is  apt  to  have  been  observed 
too  low,  the  thermometer  should  also  be  read  off,  when  the  dew  again  dis- 
appears, and  the  average  between  the  two  observations,  taken,  as  the  true 
dew-point.  Generally,  the  stand  g  on  which  this  instrument  is  supported, 
is  furnished  with  a  thermometer,  by  which  the  temperature  of  the  atmo- 
sphere at  the  same  time,  is  ascertained. 

Daniell  was  the  first  to  furnish  us  with  a  practical  hygrometer  on  a  true 
scientific  principle — that  of  finding  the  dew-point.  It  has,  however,  this 
inconvenience,  that,  as  the  cooling  of  the  ether  takes  place  from  the  upper 
surface  and  is  not  readily  communicated  to  the  layers  below,  the  thermo- 
meter is  apt  not  to  indicate  accurately  the  temperature  at  which  the  dew 
deposits.  When  the  dew-point  is  very  low,  it  is  also  difficult  to  manage, 
and  if  not  observed  at  the  moment  when  the  first  dew  appears,  which  may 
easily  escape  notice,  it  gives  the  dew-point  too  low,  and  the  experiment 
must  be  repeated.  To  facilitate  the  observation  of  the  first  dew,  the  bulb 

108 


PNEUMATICS. 


109 


made  of  dark  glass,  or  it  is  furnished  with  a  gilt  band  or  zone 
nd  it.  "  ^— *-—  ^    $MjZs^-^^ 

170.  J9acAe's  Hygrometer,  see  ^.  74,  consists  of  a  horizontal  bar  or 
Fig.  74.  tube  a  c,  of  steel  or  brass,  kept  bright  on  the 

outside,  the  one  end  of  which  is  inserted  in  a 
box  Z>,  containing  ice,  or  ice  and  salt,  by  which 
its  temperature  is  made  to  decrease  gradually 
from  the  free  end  a,  which  has  the  tempera- 
ture of  the  atmosphere,  to  the  end  c  inserted 
in  the  box.  At  the  point,  which  has  the  temperature  of  the  dew-point  of 
the  atmosphere,  the  moisture  will  begin  to  precipitate  and  form  a  very  dis- 
tinct limit,  from  which  its  amount  increases  more  and  more  toward  the 
cooled  end.  To  ascertain  accurately  the  temperature  of  the  bar,  where 
the  moisture  begins  to  precipitate,  and  which  indicates  the  dew-point,  that 
portion  of  it  which  is  outside  the  box  is  hollow,  being  varnished  inside,  if 
of  brass,  and  filled  with  mercury,  in  which  the  bulb  of  a  small  delicate 
thermometer  t  slides,  the  stem  of  which  passes  through  a  longitudinal 
Fig.  75.  Fig.  76.  opening  on  the  upper  side  of  the  bar  as 

seen  in  the  figure.  This  thermometer  is 
moved  to  the  exact  place,  where  the  mois- 
ture begins  to  condense,  and  its  tempera- 
ture then  indicates  the  dew-point.  For 
stationary  observatories,  where  ice  is  easily 
had,  this  hygrometer  is  very  convenient, 
being  easily  observed. 

171.  Regnault's  Hygrometer  (Jiygro- 
metre  condenseur)  is  a  modification  of 
Daniell' s,  but  so  contrived  as  to  be  easily  .. 
managed  and  to  give  results  of  the  utmost 
accuracy.  Fig.  75  represents  it  in  sec- 
tion. It  consists  of  a  glass  tube  h  of 
0.8  inch,  diameter,  having  on  the  side  near 
the  top  a  small  horizontal  tubulure  t.  Its  A 

lower  end  is  closed  by  being  inserted  into 
an  extremely  thin  and  highly  polished 
silver  cup  or  thimble  b  of  the  same  diame-  fJ -,'i 
ter,  and  about  If  inch,  high,  but  with  a 
round  bottom.  This  and  portion  of  the 
glass  tube  up  to  m  is  filled  with  ether,  or,  as  a  substitute,  with  alcohol.  The 
upper  end  of  the  instrument  is  closed  by  a  cork  a,  through  which  is 

109  10 


110 


BOYE'S  INANIMATE   MATTER. 


inserted  a  narrow  open  glass  tube  g,  reaching  nearly  to  the  bottom  of  the 
silver  cup,  and  a  very  accurate  thermometer  p,  the  bulb  of  which  is  in  the 
middle  of  the  ether. 

The  horizontal  tube  t  is  connected  with  an  aspirator  similar  to  g,  fig.  72, 
but  of  smaller  size,  by  which  air  may  be  drawn  with  any  desired  rapidity 
through  the  tube  g}  so  as  to  bubble  through  the  ether.  By  this  contrivance, 
the  evaporation  of  the  ether  is  under  perfect  control.  When  the  cooling 
which  it  causes  reaches  the  dew-point,  the  vapors  of  the  atmosphere  appear 
on  the  outside  of  the  silver  cup,  and  the  thermometer  is  observed.  The 
aspiration  is  then  stopped,  and  the  dew  allowed  to  disappear,  and  the 
temperature  when  this  happens,  again  observed.  The  true  dew-point  will 
then  be  the  mean  between  these  two  temperatures.  Should  it  be  desired 
to  estimate  it  with  more  accuracy,  the  aspiration  is  immediately  started 
again,  but  much  slower,  and  the  same  experiments  repeated.  By  this  con- 
trivance the  dew-point  may  be  estimated  to  -J^  of  a  degree.  To  be  better 
able  to  observe  the  slightest  dew  by  comparison  with  another  similar  appa- 
ratus, Regnault  fixes  two  such  together  by  the  tube  c  d,  which  connects 
them  both  with  the  aspirator,  as  shown  in  jig.  76,  but  the  second  of  which, 
h±  is  not  used  at  the  same  time,  and  therefore  contains  no  ether,  and  has 
the  tube  g  closed  up.  The  thermometer  of  this  maybe  used  for  indicating 
the  temperature  of  the  atmosphere. 

August's  Psychrometer  (from  <pu/pos  (psuchros),  cold,  and  //ST^OV 
(metron),  measure),  also  called  the  Thermo-Hygrometer,  but  better  known 
Fig.  77.  under  the  name  of  the    Wet-Bulb  Hygrometer,  gives 

the  dew-point  or  the  tension  of  the  vapor  in  the  atmo- 
sphere indirectly  j  from  the  greater  rapidity  with  which 
water  evaporates,  the  further  the  quantity  of  vapor  in 
the  atmosphere  is  from  the  quantity  which  would  con- 
stitute saturation.  It  consists  of  two  perfectly  similar 
thermometers,  see  5  and  a  fig.  77,  the  bulbs  of  which 
should  be  perfectly  free  in  the  air.  As  they  are  only  to 
indicate  the  temperatures  of  the  atmosphere,  the  degrees 
may  be  made  large  and  subdivided  into  fractions. 
The  bulb  of  the  one  a  is  covered  with  muslin.  The 
instrument  having  been  placed  in  the  atmosphere, 
where  exposed  to  a  free  change  of  air,  but  not  to  a 
decided  wind,  the  covered  bulb  is  thoroughly  moistened 
by  dipping  it  to  above  the  bulb  in  pure  water,  which 
should  previously  have  acquired  the  temperature  of  the 
air.  The  water  will  then  evaporate  from  the  moist- 
110 


PNEUMATICS.  HI 

ened  bulb  and  thereby  lower  its  temperature  (see  Latent  Heat  under 
Thermics),  so  that  the  quicker  the  evaporation,  the  lower  its  temperature 
will  fall.  As  soon  as  the  full  effect  is  produced,  which  generally  occurs  in 
from  5  to  10  minutes  after  its  moistening,  and  is  known  by  the  tempera- 
ture becoming  stationary,  the  thermometer  is  read  off;  the  other  thermo- 
meter is  also  read  off  at  the  same  time,  and  gives  the  temperature  of  the 
air.  Sometimes,  instead  of  dipping  the  thermometer  into  water  before 
observing  it,  it  is  kept  constantly  moistened  by  a  wick  dipping  into  a  small 
vessel  c  with  water. 

This  instrument  is  also  employed,  when  the  temperature  of  the  atmo- 
sphere is  freezing,  and  it  then  acts  by  the  cold  produced  by  the  evaporation 
of  the  layer  of  ice,  which  is  formed  round  the  bulb.  In  this  case  the 
thermometer  should  not  be  read  off  before  the  layer  of  ice  which  forms 
after  the  moistening  of  the  bulb  has  become  perfectly  dry,  which  will 
require  from  15  to  30  minutes.  It  is  then  best  to  keep  the  bulb  covered 
permanently  with  a  layer  of  ice,  which  should  be  neither  too  thin  nor  too 
thick,  since  in  both  of  these  cases  the  temperature  of  the  bulb  will  not  fall 
to  the  proper  point.  It  requires  much  care  and  practice  to  regulate  the  ? 
thickness  of  the  ice.  ' 


173.  To  obtain  the  tension  of  the  vapor  in  the  atmosphere  in  English 
inches,  from  the  indications  of  this  instrument,  we  have  the  following  for- 
mula calling  this  tension  f: 

0.480  (T—  T,) 

f  _    Z"  _  I/     f> 

1130  —  Tl 

in  which  T==  the  temperature  of  the  atmosphere  in  Fah.  degrees  given  by 
the  dry  thermometer;  ^^the  temperature  in  Fah.  degrees  of  the  wet- 
bulb  thermometer;  JF±  =  the  maximum  tension  in  English  inches  for  the 
temperature  T±  of  the  wet-bulb  thermometer,  and  which  is  found  in  Table 
IX;  and  J5  —  the  stand  of  the  Barometer  in  English  inches. 

If  the  observations  are  taken  below  32°,  when  the  wet-bulb  therefore  is 
covered  with  ice,  we  must  substitute  in  the  above  formula,  instead  of 
1130—  Tt  which  represents  the  latent  heat  of  the  vapor,  1272.2  —  T±  the 
formula  then  becoming  : 

0.480  (r-TM 


1272.2  —T' 

Having  thus  obtained  the  tension  of  the  vapor  in  the  atmosphere,  the 
relative  humidity  is  easily  calculated  (164)  by  dividing  this  tension  by  the 
maximum  tension  for  the  temperature  T  of  the  atmosphere,  given  by  the 
dry  thermometer,  and  which  tension  is  found  in  Table  IX.  If  the  dew- 
point  be  required,  it  is  easily  obtained  by  taking  from  Table  IX  the  tempe- 
rature which  corresponds  to  the  tension  /,  obtained  by  the  above  formula, 

111 


112  BOYE'S   INANIMATE   MATTER. 

174.  To  illustrate  this  by  an  example,  suppose  that  the 

Dry  Therm.  =  68°  =  T  Wet  Bulb  Therm,  =  59°  =  Tl 

Barom.  =  29.922  inch.  =  B 
We  then  have  ; 

T—  T}  =  9° 

and  from  Table  IX,          F^  =  0.500  inch. 
Therefore,  by  the  first  formula  : 

0.480x9 
/=0.500—  1130__59  X  29.  922=0.379  inch; 

which  is  therefore  the  tension  of  the  vapor  in  the  Atmosphere;  and  51°3 
which  in  Table  IX  corresponds  to  this  tension,  is  the  Dew-Point.  From 
Table  IX  we  then  obtain  : 

Max.  Tension  for  68°  =  0.685  inch. 
Therefore  : 

Relative  Humidity  =  -        r  —  .0.553; 


or  =  55.3,  if  referred  to  saturation  as  100. 

To  avoid  these  calculations,  Tables  have  been  constructed,  which  give 
from  the  temperature  T^  of  the  wet-bulb  thermometer,  and  the  temperature 
T  of  the  atmosphere  or  the  difference  T  —  T±  between  the  dry  and  wet- 
bulb  thermometers,  both  the  tension  of  the  vapor  in  the  atmosphere,  and 
the  relative  humidity,  which  at  the  temperature  T  of  the  atmosphere  cor- 
responds to  this  tension,  supposing  the  Barometer  to  remain  at  the  same  ave- 
rage stand.  If  it  should  be  required  to  make  a  correction  for  the  different 
stands  of  the  barometer,  a  table  may  also  be  constructed  for  this  purpose. 

[The  formula  given  by  August,  of  Berlin,  the  inventor  of  this  instrument,  and  which 

0.568  (t  —  t') 
is  yet  used  extensively,  is  :  «  =  /'  —  64Q_  -    h;  in  which  x  =  the  tension  of  vapor 

in  atmosphere  in  millimetres  ;  t  and  t'  =  temperatures  of  dry  and  wet  bulb  thermometers 
in  centigrade  degrees  ;  f  =  maximum  tension  of  vapors  at  temperature  if,  in  millimetres  ; 
and  h  =  stand  of  Barometer  also  in  millimetres.  By  correction  of  some  of  the  numerical 

0.429  (t—  t'  ) 
data,  Regnault  has  since  altered  this  into:  x  =/'  --  glO^T'  —  ^  which  he  found  to 

give  correct  results,  whenever  the  relative  humidity  is  less  than  0.40  (which  results  differ 
not  much  from  those  obtained  by  August's  formula,  using  August's  Table  of  Maximum 
Tensions,  but  when  taken  for  a  wider  range  are  not  so  near  the  truth).  But  whenever  the 
relative  humidity  is  over  0.40,  Regnault  has  found,  that  in  order  to  obtain  perfectly  correct 
results,  it  is  necessary  to  substitute  the  coefficient  0.480  for  0.429,  the  formula  then  becoming: 


h,  and  for  temp,  below  the  freezing  point  :*=/'—    '~     h, 

which  are  the  formulae  given  above,  only  with  the  proper  substitutions  for  using  English 
inches  and  Fahr.  degrees.     With  the   same  substitutions   August's  formula  becomes: 

0.568(7—2;)  0.429(7—7;) 

f=Fi—     1184:_y         B>  and  as  corrected  by  Regnault:  /=  Ft  —  11;j0_  y>  -  •] 

112 


c/a 


PNEUMATICS.  113 

175.  For  low  temperatures  this  instrument  gives  less  accurate  results, 
on   account  of  the   small  differences   between   the  temperatures  of  the 
dry  and  wet-bulb  thermometers ;  and  when  the  temperature  of  the  atmo- 
sphere is  near  the  freezing  point,  its  results  are  very  unsatisfactory,  on 
account  of  the  uncertainty  in  the  freezing  of  the  water.     Regnault  also 
found  that  in  order  to  obtain  good  results,  a  free  change  of  air  is  abso- 
lutely necessary,  so  that  in  a  close  room  its  indications  are  less  correct, 
the  wet-bulb  thermometer  not  descending  to  its  proper  point,  and  therefore 
giving  the  relative  humidity  too  high.     For  observations,  it  is  therefore 
generally  placed  in  an  open  window,  or  fixed  permanently  outside  of  it. 
But  even  when  thus  placed,  the  air,  if  very  still,  should  be  agitated  about 
the  bulb  by  fanning.     On  the  other  hand,  too  strong  currents  of  air  will 
affect  the  results  in  the  opposite  direction,  so  that  if  the  existing  wind 
have  a  greater  velocity  than  from  15  to  18  feet  per  second,  the  instrument 
should  be  screened  from  it.     Otherwise,  Regnault  found  that  within  the 
ordinary  limits  given  to  this  instrument,  it  is  not  influenced  perceptibly 
by  the  size  or  shape  of  the  thermometer-bulb,  nor  by  the  thickness  of  the 
covering  muslin,  nor  by  the  manner  of  moistening  it  either  by  immersion 
of  the  bulb,  or  by  supplying  it  by  a  wick  from  a  small  vessel ;  nor  in  the 
latter  case,  by  the  length  of  the  wick,  or  the  quantity  of  water  by  which 
it  is  moistened,  provided  this  be  sufficient  for  complete  moistening  and 
full  evaporation,  so  that  if  supplied  from  the  wick  in  larger  quantity  than 
this,  it  may  even  without  injury  cause  a  drop  to  fall  occasionally  from  the 
bulb,  but  in  no  case  should  it  exceed  this  quantity.     The  water  used  for 
moistening,  should  be   pure,  as   otherwise  by  its   evaporation  it  causes 
too  great  a  deposit  of  earthy  ingredients  on   the  bulb.     Rain-water  is, 
therefore,  preferable.     To  remove  impurities  which  collect  on  the  bulb, 
it  should  be  cleaned,  and  the  covering  renewed  at  least  every  two  or  three 
months. 

176.  The  Psychrometer,  from  its  simplicity  and  the  facility  with  which 
it  is  observed  and  transported,  is  almost  universally  employed  for  meteoro- 
logical observations,  both   by  travellers  and   at  stationary  observatories. 
The  above-given  precautions  and  some  practice  in  its  use,  are,  however, 
necessary  in  order  to  obtain,  reliable  results. 


• 

Hygrometers  acting  fy  absorption  of  the  vapor. 


177.  Many  organic   substances   have    the   property  of  attracting,  by 

the  force  of  adhesion,  vapor  from    the   atmosphere,  and  of  condensing 

it  on  their  surface  and  in  their  pores  (see  54),  by  which  they  increase 

their  volume  or  swell.     The  quantity  of  vapor  which  they  thus  attract  or 

H  113 


114 


BOYE'S   INANIMATE   MATTER. 


absorb,  varies  with,  the  greater  or  less  proportion,  which  the  quantity  of 
vapor  in  the  atmosphere  forms  of  the  quantity  that  would  constitute  satu- 
ration, or  in  other  words,  with  the  relative  humidity  (164),  so  that 
the  latter  to  a  certain  extent  may  be  measured  by  the  increase  or  de- 
crease of  their  volume.  Of  Hygrometers,  acting  on  this  principle,  only 
one  deserves  a  special  mention,  as  giving  results  which  approach  to  scien- 
tific accuracy,  viz. 

178.  Saussure's  Hair  Hygrometer.  It  consists  of  a  human  hair  deprived 
of  its  natural  grease  by  boiling  in  a  feeble  solution  of  carbonate  of  soda  in 
water.     This  hair  is  suspended  in  a  metallic  frame  c  Jig.  78,  the  one  end 
of  the  hair  being  attached  to  a  bracket  a,  which  may  be  adjusted  by  a 
screw;  the  other  end  is  attached  to  the  circumference  of  a  small  wheel  or 
pulley  n.     The  circumference  of  this  wheel  has  also  another  groove,  in 

Fig.  78.  which  is  fastened  and  slightly  wound  around  it  in  the  opposite 
direction,  a  thin  silk  thread,  to  which  is  attached  a  small 
weight  w,  which  therefore  constantly  keeps  the  hair  tense. 
It  will  easily  be  seen,  that  when  the  hair  by  increased 
moisture  of  the  atmosphere  absorbs  more  vapor  and  thereby 
swells,  this  will  be  perceptible  by  its  elongation,  by  which 
it  allows  the  weight  to  turn  the  wheel.  When,  on  the  con- 
trary, the  humidity  of  the  air  diminishes,  the  hair  loses  some 
of  the  condensed  vapor,  it  contracts  and  turns  the  wheel  in 
the  opposite  direction.  The  axis  of  the  wheel  carries  a 
light  index  i,  which  is  thus  made  to  traverse  a  graduated 
scale  s. 

179.  To  construct  the  scale  of  this  instrument,  it  is  first  placed  in  a 
close  receiver,  the  bottom  of  which  is  covered  with  water  and  the  sides 
moistened,  by  which  the  air  becomes  saturated  with  vapor.     The  length 
of  the  hair  having  been  so  adjusted,  that  the  index  will  then  be  near 
the  one  end  of  the  scale,  the  point  where  it  then  stands,  as  soon  as  it 
becomes  stationary,  is  marked  100°,  and  corresponds  to  saturation  or  a 
relative  humidity  of  100.      It  is  then  placed,  after  the  removal  of  the 
water,  in  the  same  close  receiver  over  oil  of  vitriol,  which  deprives  the  air 
of  all  the  vapor ;  and  the  point  on  the  scale  where  the  index  then  stands, 
after  it  has  become  almost  stationary,  is  marked  0°,  which  point  corre- 
sponds to  a  relative  humidity  of  0.     The  distance  between  0°  and  100°  is 
divided  into  100  equal  parts,  each  of  which  is  called  1  degree.     These 
degrees  do,  however,  not   correspond  to  the   same  numbers  of  relative 
humidity.     The  following  table  has  been  given  as  indicating  the  differ- 
ent  relative   humidities  corresponding   to  the   different  degrees  of  this 
hygrometer : 


PNEUMATICS. 


115 


Table  of  Relative  Humidities  corresponding  to  the  degrees  of  Saussure9 s  Hygrometer. 


ssure's  1 
rometer.l 

>  *? 

ssure's  1 
•ometei.I 

li' 

|| 

>'$ 

fj 

If 

If 

<u  £> 
'£  'H 

ssure's  1 
roineter.l 

•£'•3 

if 

it 

•oineter.| 

II 

ssure's  1 
rometer.l 

If 

ssure  s  1 
rometer  1 

If 

H 

H 

II 

^a 

«! 

1| 

H 

II 

II 

II 

«I 

«l 

§1 

•a  a 

l| 

II 

2| 

l| 

*l 

•  0° 

0 

10° 

5 

20° 

12 

30° 

19 

40° 

"if 

80° 

35 

60° 

44 

70° 

56 

80° 

69 

90° 

83 

1° 

0 

11° 

6 

21° 

12 

31° 

20 

41° 

27 

51° 

36 

61° 

45 

71° 

57 

81° 

70 

91° 

85 

2° 

1 

12° 

6 

22° 

13 

32° 

21 

42° 

28 

52° 

37 

62° 

46 

72° 

58 

82° 

72 

92° 

87 

3° 

1 

13° 

7 

23° 

14 

33° 

22 

48° 

28 

53° 

37 

63° 

47 

73° 

59 

83° 

73 

93° 

88 

4° 

2 

14° 

8 

24° 

15 

34° 

23 

44° 

29 

54° 

38 

64° 

49 

74° 

61 

84° 

-9fr 

94° 

90 

5° 

3 

15° 

8' 

S£>° 

16 

35° 

24 

45° 

30 

55° 

39 

65° 

^ 

^5° 

62 

85° 

77 

95° 

91 

6° 

3 

16° 

9 

26° 

17 

36° 

24 

46° 

31 

56° 

40 

66° 

51 

76° 

63 

86° 

78 

96° 

93 

7° 

4 

17° 

10 

27° 

18 

37°- 

•&r 

47° 

32 

57° 

41 

67° 

52 

77° 

65 

87° 

79 

97° 

95 

8° 

4 

18° 

11 

28° 

18 

38° 

26 

48° 

33 

58° 

42 

68° 

53 

78° 

66 

88° 

81 

98° 

97 

9° 

5 

19° 

11 

29° 

19 

39° 

26 

49° 

34 

59° 

43 

09° 

55 

79° 

68 

89° 

82 

99° 

98 

180.  But  this  instrument  is  not  so  uniform,  that  the  above  comparison 
can  be  relied  on. 

Saussure's  directions  (Essais  surl'Hygrometrie,  par  B.  H.  de  Saussure,  Neufchatel,  1783) 
are :  to  select  fine,  soft,  not  curly,  nor  splitting  hair,  cut  from  the  head  of  a  living  and  sane 
person.  A  bunch  of  these  of  the  thickness  of  a  quill  is  then  sewed  up  between  linen,  sepa- 
rating them  as  much  as  possible.  They  are  then  boiled  for  30  minutes  in  a  solution  of 
154  grains  of  Crystallized  Carbonate  of  Soda  in  32  oz.  (Troy)  of  Water,  which  should  be 
performed  in  a  flask  with  a  long  neck,  to  prevent  the  evaporation  of  the  water.  The  bag 
is  then  twice  boiled  for  a  few  minutes  in  pure  water,  cut  open,  and  the  hairs  again  washed 
and  separated  by  moving  them  to  and  fro  in  a  large  vessel  with  cold  water,  after  which 
they  are  dried  in  the  open  air.  The  hairs  should  appear  clean,  soft,  polished  and  trans- 
parent, separating  easily  from  each  other.  If  they  are  rough  and  adhere,  they  have  either 
been  boiled  too  long,  or  the  solution  has  become  too  strong  by  the  evaporation  of  the  water. 
The  length  of  the  hair  in  the  frame  should  be  about  9^  inch. ;  the  diameter  of  the  pulley 
on  which  it  acts  0.2  inch.  The  index  should  be  light,  and  with  the  pulley  perfectly  balanced 
by  itself.  The  extending  weight  should  not  exceed  3  grains;  if  increased  to  only  9  grains, 
the  instrument  will,  after  some  time,  work  irregularly.  Saussure  has  also  studied  the  influ- 
ence of  the  temperature  on  it,  and  gives  a  table  for  reducing  its  indications  to  the  same 
temperature.  He  asserts  that  if  his  directions  are  strictly  adhered  to>  the  instrument  will 
never  vary  more  than  2  to  3  degrees.  By  later  experiments,  Regnault  found  no  greater 
difference  with  the  same  kind  of  hair,  if  prepared  in  one  and  the  same  operation ;  with 
different  kinds  of  hair,  also  prepared  in  the  same  operation,  the  difference  amounted  to 
nearly  5°;  about  the  same  difference  (5°)  was  produced  with  the  same  kind  of  hair,  and 
prepared  in  the  same  operation,  but  with  small  differences  in  the  weights  by  which  the  hairs 
were  extended.  But  icith  different  kinds  of  hair,  and  prepared  in  different  operations,  and 
having  been  in  use  for  different  lengths  of  time,  the  differences  amounted  in  some  cases  to  15°, 
even  after  the  extreme  points  of  the  scale  had  been  fixed  correctly.  Regnault  concludes  from 
this,  that  it  is  necessary  to  construct  a  table  for  every  instrument,  by  comparing  its  degrees 
with  known  relative  humidities  of  the  air,  which  he  produces  by  placing  it  in  a  close 
receiver  with  different  mixtures  of  oil  of  vitriol  and  water,  for  which  mixtures  he  has  given 
an  elaborate  table  of  tensions ;  and  also  to  test  the  instrument  from  time  to  time  when 
in  use.  He  also  proposes  to  remove  the  natural  grease  by  placing  the  hairs  for  24 
hours  in  ether,  by  which  they  retain  their  strength  and  solidity,  and  acquire  almost  the 
same  sensibility.  As  by  placing  the  instrument  in  perfectly  dry  air  in  order  to  fix  the  0°, 
it  requires  several  days  to  become  moderately  stationary,  and  the  hair  continues  to  con- 
US 


116  BOYE'S  INANIMATE  MATTER. 

tract,  though  much  less,  even  for  several  months,  he  considers  the  state  to  which  it  is  thus 
reduced  as  unnatural,  and  therefore  permanently  injuring  its  hygrometrical  properties. 
As  the  air  also  never  reaches  this  degree,  he  proposes,  therefore,  to  drop  the  present  0° 
altogether,  and  to  begin  the  scale  from  a  point,  which  corresponds  to  a  relative  humidity 
of  20°,  and  which  is  produced  in  a  close  receiver  at  the  temperature  of  42°83  Fah.  by  the 
mixture  of  oil  of  vitriol  and  water,  which  has  the  chemical  composition  of  1  atom  of  sul- 
phuric acid,  and  5  atoms  of  water,  being  represented  by  the  formula,  S03  +  5  HO. 


". 


181.  gyroscopes.  Many  other  instruments  have  been  constructed  from 
other  organic  substances,  acting  on  the  same  principle  as  the  hair  hygrome- 
ter; but  all  these  have  no  scientific  value  whatever,  as  none  of  them  can  be 
relied  upon  for  indicating  the  relative  humidity,  even  only  approximately. 
They  are  therefore  not  hygrometers,  but  horoscopes  (from  UYP°S  (hugros), 
and  ffxoxscu  (skopeo),  I  observe),  and  as  such  they  may  be  used  with  advan- 
tage to  indicate  a  mere  increase  or  decrease  in  the  moisture  of  the  atmo- 
sphere. Of  such  may  be  mentioned,  strips  or  bars  of  whalebone  or  wood, 
cut  across  the  grain.  The  former  may  be  reduced  to  a  thin  thread  or  band, 
and  may  be  made  to  act  on  a  wheel  with  an  index  in  a  similar  manner  as 
the  hair.  All  twisted  strings  made  of  vegetable  fibres,  as  hempen  cords, 
or  of  animal  membranes,  as  cat-gut  or  violin  strings,  will  swell  by  moisture 
and  thus  by  the  increase  in  their  diameter  untwist  themselves,  or,  if  pre- 
vented from  this,  become  shorter  by  the  increased  twist.  A  piece  of  violin 
string,  if  properly  prepared,  may  thus,  by  its  untwisting,  be  made  to  turn 
back  the  hood  or  cowl  from  the  head  of  a  figure  in  dry  weather  and  to 
replace  it  in  damp  weather;  or  to  raise  its  arm  and  unfurl  an  umbrella; 
or  to  turn  a  lever  so  as  to  show  alternately  through  a  window  or  before  the 
door  of  a  toy-house,  two  different  figures,  representing  rainy  and  fine  weather. 
The  beard  of  the  husk  around  the  seed  of  Sensitive  Oats  (Avena  sensitiva), 
is  naturally  twisted  or  coiled  as  a  double  spiral,  so  that  if  one  end  be 
fastened  in  the  centre  of  a  graduated  circle,  and  a  light  index  of  straw 
attached  by  sealing-wax  to  the  other,  the  latter  will  traverse  the  circular 
scale  by  the  coiling  or  uncoiling  of  the  beard  by  the  moisture  in  the  air. 
The  bladder  of  a  rat  or  squirrel,  may  also  be  converted  into  a  hygroscope, 
by  tying  its  mouth  over  the  end  of  an  open  glass-tube  and  filling  the  bladder 
and  part  of  the  tube  with  mercury.  By  the  contraction  or  swelling  of  the 
bladder  by  the  change  in  moisture,  the  mercury  will  rise  or  fall  in  the  tube. 
Whalebone,  reduced  to  the  thinness  of  fine  paper,  goldbeater's  skin,  and 
thin  sheets  of  gelatine  or  glue,  will  show  such  sensitiveness  to  moisture, 
that  if  cut  into  figures,  as  fishes,  etc.,  and  placed  in  the  palm  of  the  hand, 
the  natural  moisture  of  the  latter  will  cause  the  side  next  to  it  to  swell, 
and  the  figure  to  curl  itself  up. 


116 


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TABLE  II.   Correction  for  reducing  Observed  Height  of  Barom.  to  Stand.  Temp,  of  32°  Fah. 
The  Scale  being  of  brass  and  extending  the  whole  length  of  instrument.  Formula  in  note  to  par.  77,  p.  49. 


Observed 
Temp,  of 
Barom. 
Fah. 

Observed  Height  in  English  Inches. 

26.5 

27 

27.5 

2S 

28.5 

29 

29.5 

30 

30.5 

31 

0° 

+.068 

+.069 

+.071 

+.072 

+.073 

+.074 

+.076 

+.077 

+.078 

+.080 

1 

.065 

.067 

.068 

.069 

.071 

.072 

.073 

.074 

.076 

.077 

2 

.063 

.064 

.066 

.067 

.068 

.069 

.070 

.072 

.073 

.074 

3 

.061 

.062 

.063 

.064 

.065 

.067 

.068 

.069 

.070 

.071 

4 

.058 

.059 

.061 

.062 

.063 

.064 

.065 

.066 

.067 

.068 

5 

.056 

.057 

.058 

.059 

.060 

.061 

.062 

.063 

.065 

.066 

6 

+.054 

+.055 

+.056 

+.057 

+.058 

+.059 

+.060 

+.061 

+.062 

+.063 

7 

.051 

.052 

.053 

.054 

.055 

.056 

.057 

.058 

.059 

.060 

8 

.049 

.050 

.051 

.052 

.053 

.054 

.054 

.055 

.056 

.057 

9 

.046 

.047 

.048 

.049 

.050 

.051 

•052 

.053 

.054 

.054 

10 

.044 

.045 

.046 

.047 

.047 

.048 

.049 

.050 

.051 

.052 

11 

+.042 

+.042 

+  .043 

+.044 

+.045 

+.046 

+.046 

+.047 

+.048 

+.049 

12 

.039 

.040 

.041 

.042 

.042 

.043 

.044 

.045 

.045 

.046 

13 

.037 

.038 

.038 

.039 

.040 

.040 

.041 

.042 

.043 

.043 

14 

.035 

.035 

.036 

.037 

.037 

.038 

.038 

.039 

.040 

.040 

15 

.032 

.033 

.033 

.034 

.035 

.035 

.036 

.036 

.037 

.038 

16 

+.030 

+.030 

+.031 

+.032 

+.032 

+.033 

+.033 

+.034 

+.034 

+.035 

17 

.027 

.028 

.028 

.029 

.030 

.030 

.031 

.031 

.032 

.032 

18 

.025 

.025 

.026 

.026 

.027 

.027 

.028 

.028 

.029 

.029 

19 

.023 

.023 

.024 

.024 

.024 

.025 

.025 

.026 

.026 

.027 

20 

.020 

.021 

.021 

'.021 

.022 

.022 

.023 

.023 

.023 

.024 

21 

22 

+.018 
.016 

+.018 
.016 

+.019 
.016 

4-.019 
.016 

-L.019 
.017 

+.020 
.017 

+.020 
.017 

+.020 
.018 

+.021 
.018 

+.021 

.018 

23 

.013 

.013 

.014 

.014 

.014 

.014 

.015 

.015 

.015 

.015 

.  24 

.011 

.011 

.011 

.011 

.012 

.012 

.012 

.012 

.012 

.013 

25 

.008 

.009 

.009 

.009 

.009 

.009 

.009 

.009 

.010 

.010 

26 

27 

+.006 
.004 

+.006 
.004 

+.006 
.004 

+.006 
.004 

+.006 
.004 

+.007 
.004 

+.007 
.004 

+.007 
.004 

+.007 
.004 

+.007 
.004 

28 

.001 

.001 

.001 

.001 

.001 

.001 

.001 

.001 

.001 

.001 

29 

—.001 

—.001 

—.001 

—.001 

—.001 

—.001 

—.001 

—.001 

—.001 

001 

30 

.004 

.004 

.004 

.004 

.004 

.004 

.004 

.004 

.004 

.004 

31 

—.006 

—.006 

—.006 

—.006 

—.006 

—.007 

—.007 

—.007 

007 

—.007 

32 

.008 

.008 

.009 

.009 

.009 

.009 

.009 

•009 

.010 

.010 

33 

.011 

.011 

.011 

.011 

.012 

.012 

.012 

.012 

.012 

.012 

34 

.013 

.013 

.014 

.014 

.014 

.014 

.015 

.015 

.015 

.015 

35 

.015 

.016 

.016 

.016 

.017 

.017 

.017 

.018 

.018 

.018 

36 

—.018 

-.018 

—.019 

—.019 

—.019 

—.020 

—.020 

—.020 

—•021 

—.021 

37 

.020 

.021 

.021 

.021 

.022 

.022 

.022 

.023 

.023 

.024 

38 

.023 

.023 

.023 

.024 

.024 

.025 

.025 

.026 

.026 

.026 

39 

.025 

.025 

.026 

.026 

.027 

.027 

.028 

.028 

.029 

.029 

40 

.027 

.028 

.028 

.029 

.029 

.030 

.030 

.031 

.031 

.032 

41 

—.030 

—.030 

—.031 

—.031 

—.032 

—.033 

—.033 

—.034 

—.034 

—.035 

42 

.032 

.033 

.033 

.034 

.034 

.035 

.036 

.036 

.037 

.037 

43 

.034 

.035 

.036 

.036 

.037 

.038 

.038 

.034 

.040 

.040 

44 

.037 

.037 

.038 

.039 

.040 

.040 

.041 

.042 

.042 

.043 

45 

,039 

.040 

.041 

.041 

.042 

.043 

.044 

.044 

.045 

.046 

46 

—.042 

—.042 

—.043 

—.044 

—.045 

—.045 

—.046 

—.047 

—.048 

—.049 

47 

.044 

.045 

.046 

.046 

.047 

.048 

.049 

.050 

.051 

.051 

48 

.046 

.047 

.048 

.049 

.050 

.051 

.052 

.052 

.053 

.054 

49 

.049 

.050 

.050 

.051 

.052 

.053 

.054 

.055 

.056 

.057 

50 

.051 

.052 

.053 

.054 

.055 

.056 

.057 

.058 

.059 

.060 

TABLE  II.  Correction  for  reducing  Observed  Height  of  Barom.  to  Stand.  Temp,  of  32°  Fah. 
Tha  Scale  being  of  brass  and  extending  the  whole  length  of  instrument.    Formula  in  note  to  par.  77,  p.  49. 


Observed 
Temp,  of 
Baroin. 
Fall. 

Observed  Height  In  Knglish  Inches. 

26.5 

27 

27.5 

28 

28.5 

29 

29.5 

30 

30.5 

31 

51° 

—.053 

—.054 

—.055 

—.056 

—.057 

—.058 

—.059 

—.060 

—.061 

—.062 

52 

.056 

.057 

.058 

.059 

.060 

.061 

.062 

.063 

.064 

.065 

53 

.058 

.059 

.060 

.061 

.063 

.064 

.065 

.066 

.067 

.068 

54 

.060 

.062 

.063 

.064 

.065 

.066 

.067 

.068 

.070 

.071 

55 

.063 

.064 

.065 

.066 

.068 

.069 

.070 

.071 

.072 

.073 

56 

—.065 

—.066 

—.068 

—.069 

—.070 

—.071 

—.073 

—.074 

—.075 

—.076 

57 

.068 

.069 

.070 

.071 

.073 

.074 

.075 

.076 

.078 

.079 

58 

.070 

.071 

.073 

.074 

.075 

.077 

.078 

.079 

.081 

.082 

59 

.072 

.074 

.075 

.076 

.078 

.079 

.080 

.082 

.083 

.085 

60 

.075 

.076 

.077 

.079 

.080 

.082 

.083 

.085 

.086 

.087 

61 

—.077 

—.078 

-.080 

—.081 

—.083 

—.084 

—.086 

—.087 

—.089 

—.090 

62 

.079 

.081 

.082 

.084 

.085 

.087 

.088 

.090 

.091 

.093 

63 

.082 

.083 

.085 

.086 

.088 

.089 

.091 

.093 

.094 

.096 

64 

.084 

.086 

.087 

.089 

.090 

.092 

.094 

.095 

.097 

.098 

65 

.086 

.088 

.090 

.091 

.093 

.095 

.096 

.098 

.100 

.101 

66 

—.089 

—.090 

—.092 

—.094 

—.096 

—.097 

—.099 

—.101 

—.102 

—.104 

67 

.091 

.093 

.095 

.096 

.098 

.100 

.102 

.103 

.105 

.107 

68 

.094 

.095 

.097 

.099 

.101 

.102 

.104 

.106 

.108 

.109 

69 

.096 

.098 

.100 

.101 

.103 

.105 

.107 

.109 

.110 

.112 

70 

.098 

.100 

.102 

.104 

.106 

.108 

.109 

.111 

.113 

.115 

71 

—.101 

—.102 

—.104 

—.106 

—.108 

—.110 

—.112 

114 

—.116 

—  118 

72 

.103 

.105 

.107 

.109 

.111 

.113 

.115 

.117 

.119 

.120 

73 

.105 

.107 

.109 

.111 

.113 

.115 

.117 

.119 

.121 

.123 

74 

.108 

.110 

.112 

.114 

.116 

.118 

.120 

.122 

.124 

.126 

75 

.110 

.112 

.114 

.116 

.118 

.120 

.122 

.125 

.127 

.129 

76 

—.112 

—.114 

—.117 

—.119 

—.121 

—  123 

—.125 

_.127 

—.129 

—.131 

77 

.115 

.117 

.119 

.121 

.123 

.126 

.128 

.130 

.132 

.134 

78 

.117 

.119 

.122 

.124 

.126 

.128 

.130 

.133 

.135 

.137 

79 

.118 

.122 

.124 

.126 

.128 

.131 

.133 

.135 

.137 

.140 

80 

.122 

.124 

.126 

.129 

.131 

.133 

.136 

.138 

.140 

.143 

81 

—.124 

—.126 

—.129 

—.131 

—.134 

—.136 

—.138 

141 

—.143 

—.145 

82 

.126 

.129 

.131 

.134 

.136 

.138 

.141 

.143 

.146 

.148 

83 

.129 

.131 

.134 

.136 

.139 

.141 

.143 

.146 

.148 

.151 

84 

.131 

.134 

.136 

.139 

.141 

.144 

.146 

.149 

.151 

.154 

85 

.133 

.136 

.139 

.141 

.144 

.146 

.149 

.151 

.154 

.156 

86 

_.136 

—.138 

—.141 

—.144 

—.146 

—.149 

—.151 

—.154 

—.156 

—.159 

87 

.138 

.141 

.143 

.146 

.149 

.151 

.154 

.157 

.159 

.162 

88 

.141 

.143 

.146 

.149 

.151 

.154 

.157 

.159 

.162 

.165 

89 

.143 

.146 

.148 

.151 

.154 

.156 

.159 

.162 

.165 

.167 

90 

.145 

.148 

.151 

.153 

.156 

.159 

.162 

.164 

.167 

.170 

91 

_.148 

—.150 

—.153 

—.156 

—.159 

—.162 

—.165 

—  167 

—.170 

—.173 

92 

.150 

.153 

.156 

.158 

.161 

.164 

.167 

.170 

.172 

.175 

93 

.152 

.155 

.158 

.161 

.164 

.167 

.170 

.172 

.175 

.178 

94 

.155 

.157 

.161 

.163 

.166 

.169 

.172 

.175 

.177 

.180 

95 

.157 

.160 

.163 

.166 

.169 

.172 

.175 

.178 

.180 

.183 

96 

—159 

—.162 

—.165 

—.168 

—.171 

—.174 

—.178 

—.181 

-.183 

—.186 

97 

.162 

.165 

.168 

.171 

.174 

.177 

.180 

.183 

.186 

.189 

98 

.164 

.167 

.170 

.173 

.176 

.179 

.183 

.186 

.188 

.191 

99 

.166 

.169 

.173 

.176 

.179 

.182 

.185 

.188 

.191 

.194 

100 

.169 

.172 

.175 

.178 

.181 

.184 

.188 

.191 

.194 

.197 

TABLE   III.    Giving    the   different    distances  from    the    uppermost     accessible    limit   of 

the  Atmoa.  (5.7  miles)  corresponding  to  the  different  heights  of  the  Barom. 

The  temp,  of  the  Atmosphere  being  32°.    See  Pn.  par.  95,  page  67.* 


3arom. 
inches. 

Distances 
in  Feet. 

Diff. 

with  pro- 
portional 

Barom. 
[nches. 

Distances 
in  Feet. 

Diff. 

with  pro- 
portional 
parts  for 
thou- 
sandths 
of  Inches 

Barom. 
Inches. 

Distances 
in  Feet. 

Diff. 

with  pro- 
portional 
parts  for 
thou- 
sandths 
of  Inches. 

28.00 
.01 
.02 
.03 
.04 
.05 
.06 
.07 
.08 
.09 

28.10 
.11 
.12 
.13 
.14 
.15 
.16' 
.17 
.18 
.19 

28.20 
.21 
.22 
.23 
.24 
.25 
.26 
.27 
.28 
.29 

28.30 
.31 
.32 
.33 
.34 
.35 
.36 
.37 
.38 
.39 

28.40 
.41 
.42 
.43 
.44 
.45 
.46 
.47 
.48 
.49 

27425-3 
27434-6 
27444-0 
27453-3 
27462-6 
27471-9 
27481-3 
27490-6 
27499-9 
27509-2 

27518-4 
27527-7 
27537-0 
27546-3 
27555-6 
27564-9 
27574-2 
27583-5 
27592-7 
27602-0 

27611-3 
27620-6 
27629-8 
27639-1 
27648-3 
27657-6 
27666-8 
27676-1 
27685-3 
27694-6 

27703-7 
27712-9 
27722-2 
27731-4 
27740-6 
27749-8 
27759-1 
27768-3 
27777-5 
27786-7 

27795-8 
27805-0 

27814-2 
27823-4 
27832-6 
27841-8 
27851-0 
27860-2 
27869-3 
27878-5 

parts  for 
thou- 
sandths 
of  laches 

28.50 
.51 
.52 
.53 
.54 
.55 
.56 
.57 
.58 
.59 

28.60 
.61 
.62 
.63 
.64 
.65 
.66 
.67 
.68 
.69 

28.70 
.71 

.72 
.73 
.74 
.75 

.76 

.77 
.78 
.79 

28.80 
.81 
.82 
.83 
.84 
.85 
.86 
.87 
.88 
.89 

28.90 
.91 
.92 
.93 
.94 
.95 
.96 
.97 
.98 
.99 

27887-7 
27896-9 
27906-0 
27915-2 
27924-3 
27933-5 
27942-6 
27951-8 
27960-9 
27970-1 

27979-2 
27988-3 
27997-5 
28006-6 
28015-7 
28024-8 
28034-0 
28048-1 
28052-2 
28061-3 

28070-5 

28079-6 
28088-7 
28097-8 
28106-9 
28115-9 
28125-0 
28134-1 
28143-2 
28152-2 

28161-3 
28170-4 
28179-4 
28188-5 
28197-5 
28206-6 
28215-6 
28224-7 
28233-7 
28242-8 

28251-8 
28260-8 
28269-9 
28278-9 
28287-9 
28296-9 
28306-0 
28315-0 
28324-0 
28333-0 

29.00 
.01 
.02 
.03 
.04 
.05 
.06 
.07 
.08 
.09 

29.10 
.11 
.12 
.13 
.14 
.15 
.16 
.17 
.18 
.19 

29.20 
.21 
.22 
.23 
.24 
.25 
.26 
.27 
.28 
.29 

29.30 
.31 
.32 
.33 
.34 
.35 
.36 
.37 
.38 
.39 

29.40 
.41 
.42 
.43 
.44 
.45 
.46 
.47 
.48 
.49 

28342-1 
28351-1 
28360-1 
28369-1 
28378-1 
28387-1 
28396-1 
28405-0 
28414-0 
28423-0 

28432-0 
28441-0 
28450-0 
28458-9 
28467-9 
28476-9 
28485-8 
28494-8 
28503-8 
28512-7 

28521-7 
28530-6 
28539-6 
28548-5 
28557-5 
28566-4 
28575-4 
28584-3 
28593-2 
28602-2 

28611-1 
28620-0 
28628-9 
28637-8 
28646-7 
28655-6 
28664-5 
28673-4 
28682-3 
28691-2 

28700-0 

28708-9 
28717-8 
28726-6 
28735-5 
28744-4 
28763-3 
28762-1 
28771-0 
28779-9 

9.4 

9.1 

9.0 

1 

2 
3 
4 
5 

6 

7 
8 
9 

0.9 
1.9 

2.8 
3.8 
4,7 
5.6 
6.6 
7.5 
8.5 

1 

2 
3 
4 
5 
6 
7 
8 
9 

0.9 
1.8 
2.7 
3.6 
4.6 
5.5 
6.4 
7.3 
8.2 

1 

2 
3 
4 
5 
6 

N 

8 
8 

0.9 
1.8 
2.7 
3.6 
4.5 
5.4 
6.3 
7.2 
8.1 

9.3 

9.0 

8.9 

1 

2 
3 

4 

5 
6 
7 
8 
9 

0.9 
1.9 
2.8 
3.7 
4.7 
5.6 
6.5 
7.4 
8.4 

9.2 

1 
2 

3 
4 
5 
6 
7 
8 
9 

0.9 
1.8 
2.7 
3.6 
4.5 
5.4 
6.3 
7.2 
8.1 

1 

2 
8 

4 
5 
6 

7 
8 
9 

0.9 
1.8 
2.7 
3.6 
4.5 
5.5 
6.2 
7.1 
8.0 

1 
2 
8 
4 
5 
8 
7 
8 
0 

0.9 
1.8 
2.8 
3.7 
4.6 
5.5 
6.4 
7.4 
8.3 

*  The  distances  in  this  Table  have  been  obtained  by  deductini 
refer  to  the  upper  sensible  limit  (about  17  miles  above  the  earth). 

4 


59633.6  feet  from  those  given  by  the  formula,  which 


TABLE  III  (Continued).  Giving  the  different  distances  from  the  uppermost  accessible  limit  of 

the  Atmo*.  (5.7  miles)  corresponding  to  the  different  heights  of  the  Barom. 

The  temp,  of  the  Atmosphere  being  32°.    See  Pn.  par.  95,  page  67.* 


Barom. 
Inches. 

Distances 
in  Feet. 

Diff. 

with  pro- 
portional 
parts  for 
thou- 
sandths 
of  Inch. 

Barom. 
Inches. 

Distances 
in  Feet. 

Diff. 

with  pro- 
portional 
parts  for 
thou- 
sandths 
of  Inch. 

Barom. 
Inches. 

Distances 
in  Feet. 

Diff. 

with  pro- 
portional 
parts  for 
thou- 
sandths 
of  Inch. 

29.50 
.51 
.52 
.53 
.54 
.55 
.56 
.57 
.58 
.59 

29.60 
.61 
.62 
.63 
.64 
.65 
.66 
.67 
.68 
.69 

29.70 
.71 
.72 
.73 
.74 
.75 
.76 
.77 
.78 
.79 

29.80 
.81 
.82 
.83 
.84 
.85 
.86 
.87 
.88 
.89 

29.90 
.91 
.92 
.93 
.94 
.95 
.96 
.97 
.98 
.99 

28788-7 
28797-5 
28806-4 
28815-2 
28824-1 
28832-9 
28841-8 
28850-6 
28859-4 
28868-3 

28877-1 
28885-9 
28894-7 
28903-6 
28912-4 
28921-2 
28930-0 
28938-8 
28947-6 
28956-4 

28965-2 
28974-0 
28982-8 
28991-6 
29000-4 
29009-1 
29017-9 
29026-7 
29035-5 
29044-2 

29053-1 
29061-9 
29070-6 
29079-4 
29088-1 
29096-9 
29105-6 
29114-4 
29123-1 
29131-9 

29140-6 
29149-3 
29158-1 
29166-8 
29175-5 
29184-2 
29193-0 
29201-7 
29210-4 
29219-1 

30.00 
.01 
.02 
.03 
.04 
.05 
.06 
.07 
.08 
.09 

30.10 
.11 
.12 
.13 
.14 
.15 
.16 
.17 
.18 
.19 

30.20 
.21 
.22 
.23 
.24 
.25 
.26 
.27 
.28 
.29 

30.30 
.31 
.32 
'.83 
.34 
.35 
.36 
.37 
.38 
.39 

30.40 
.41 
.42 
.43 
.44 
.45 
.46 
.47 
.48 
49 

29227-8 
29236-5 
29245-2 
29253-9 
29262-6 
29271-3 
29280-0 
29288-7 
29297-3 
29306-0 

29314-7 
29323-4 
29332-0 
29340-7 
29349-3 
29358-0 
29366-7 
29375-3 
29384-0 
29392-6 

29401-3 
29409-9 
29418-6 
29427-2 
29435-9 
29444-5 
29453-2 
29461-8 
29470-4 
29479-1 

29487-7 
29496-3 
29504-9 
29513-6 
29522-2 
29530-8 
29539-4 
29548-0 
29556-6 
29565-2 

29573-8 
29582-4 
29591-0 
29599-6 
29608-2 
29616-7 
29625-3 
29633-9 
29642-5 
29651-0 

30.50 
.51 
.62 
.53 
.54 
.55 
.56 
.57 
.58 
.59 

30.60 
.61 
.62 
.63 
.64 
.65 
.66 
.67 
.68 
.69 

30.70 
.71 
.72 
.73 
.74 
.75 
.76 
.77 
.78 
.79 

£0.80 
.81 
.82 
.83 
.84 
.85 
.86 
.87 
.88 
.89 

30.90 
.91 
.92 
.93 
.94 
.95 
.96 
.97 
.98 
.99 

29659-6 
29668-1 
29676-7 
29685-2 
29693-8 
29702-3 
29710-9 
29719-4 
29727-9 
29736-5 

29745-0 
29753-5 
29762-1 
29770-6 
29779-1 
29787-6 
29796-2 
29804-7 
29813-2 
29821-7 

29830-2 

29838-7 
29847-2 
29855-7 
29864-2 
29872-7 
29881-2 
29889-7 
29898-2 
29906-7 

29915-2 
29923-7 
29932-2 
29940-7 
29949-2 
29957-6 
29966-1 
29974-6 
29983-5 
29991-1 

30000-0 
30008-5 
30016-9 
30025-4 
300338 
30042-3 
30050-7 
30059-2 
30067-6 
30076-1 

8.9 

8.7 

8.6 

1 

2 
3 
4 
5 
6 
7 
8 
9 

0.9 
1.8 
2.7 
3.6 
4.5 
5.3 
6.2 
7.1 
8.0 

1 

2 
3 
4 
5 

6 
7 
8 
9 

0.9 
1.7 
2.6 
3.5 
4.4 
5.2 
6.1 
7.0 
7.8 

1 

2 

3 
4 
5 
6 
7 
8 
9 

0.9 
1.7 
2.6 
3.4 
4.3 
5.2 
6.0 
6.9 
7.7 

8.8 

8.6 

8.5 

1 

2 
3 
4 
6 

6 

7 

8 

9 

0.9 
1.8 
2.6 
3.5 
4.4 
5.3 
6.2 
7.0 
7.9 

1 
2 
8 

4 
5 
6 

7 
8 
9 

0-9 
1.7 
2.6 
3.4 
4.3 
5.1 
6.0 
6.8 
7.7 

8.7 

8.4 

1 

2 
& 
.( 
B 
(3 
7 
8 
I 

0.9 
1.7 
2.6 
3.5 
4.4 
5.2 
6.1 
7.0 
7.8 

] 

2 
3 

4 
5 
6 
7 
8 
9 

0.9 
1.7 
2.6 
3.4 
4.3 
5.2 
6.0 
6.9 
7.7 

1 
2 
3 

4 
5 

(i 
7 

s 

y 

0.8 
1.7 
2.5 
3.4 
4.2 
5.0 
5.9 
6.7 
7.6 

*  To  avoid  too  large  numbers,  the  distances  in  this  Table  have  not  been  referred  to  the  upper  sensible  limit  (17  miles), 
but  to  the  uppermost  accessible  limit  (5.7  miles),  by  deducting  5%:)3.6  feet  from  those  obtained  by  the  indicated  method. 

5 


TABLE  IV.  Correction  for  Latitude,  on  account  of  Decrease  of  Gravity  from  Pole  to  Equator. 

To  be  applied  to  Height  obtained  from  Barometric  Observations,  see  par.  95,  page  68. 

ADD  this  correction  if  Lat.  less  than  43°  ;  DEDUCT  if  Lat.  greater  than  45°. 


Latitude. 

Obtained  Height. 

1  Foot. 

2  Feet. 

3  Feet. 

4  Feet. 

5  Feet. 

6  Feet. 

7  Feet. 

8  Feet. 

J  Feet. 

Thou- 

Thou- 

Thou- 

Thou- 

Thou- 

Thou- 

Thou- 

Thou- 

Thou- 

Deduct 

sandths 

sandths 

sandths 

sandths 

sandths 

sandths 

sandths 

sandths 

sandths 

kdu  for. 

for. 

of  Feet. 

of  Feet. 

of  Feet. 

of  Feet. 

of  Feet. 

of  Feet. 

of  Feet. 

of  Feet. 

of  Feet. 

0° 

90° 

2.837 

5.674 

8.511 

11.348 

14.186 

17.023 

19.860 

22.697 

25.534 

1° 

89° 

2.835 

5.671 

8.506 

11.342 

14.177 

17.012 

19.848 

22.683 

25.518 

2° 

88° 

2.830 

5.660 

8.491 

11.321 

14.151 

16.981 

19.811 

22.642 

25.472 

3° 

87° 

2.822 

5.643 

8.465 

11.286 

14.108 

16.929 

19.751 

22.573 

25.394 

4° 

86° 

2.810 

5.619 

8.429 

11.238 

14.047 

16.857 

19.666 

22.476 

25.285 

5° 

85° 

2.794 

5.588 

8.382 

11.176 

13.970 

16.764 

19.558 

22.352 

25.146 

6° 

84° 

2.775 

5.550 

8.325 

11.100 

13.876 

16.651 

19.426 

22.201 

24.976 

7° 

83° 

2.753 

5.506 

8.259 

11.011 

13.764 

16.517 

19.270 

22.023 

24.775 

8° 

82° 

2.727 

5.454 

8.182 

10.909 

13.636 

16.363 

19.090 

21.818 

24.545 

9° 

81° 

2.698 

5.397 

8.095 

10.793 

13.491 

16.190 

18.888 

21.586 

24.284 

10° 

80° 

2.666 

5.332 

7.998 

10.664 

13.330 

15.996 

18.662 

21.328 

23.994 

11° 

79° 

2.631 

5.261 

7.892 

10.522 

13.153 

15.783 

18.413 

21.044 

23.675 

12° 

78° 

2.592 

5.184 

7.776 

10.367 

12.959 

15.551 

18.142 

20.735 

23.326 

13° 

77° 

2.550 

5.100 

7.650 

10.200 

12.750 

15.300 

17.850 

20.400 

22.950 

14° 

76° 

2.505 

5.010 

7.515 

10.020 

12.525 

15.033 

17.535 

20.040 

22.545 

15° 

75° 

2.457 

4.914 

7.371 

9.828 

12.285 

14.742 

17.199 

19.656 

22.113 

16° 

74° 

2.406 

4.812 

7.218 

9.624 

12.030 

14.436 

16.842 

19.248 

21.654 

17° 

73° 

2.352 

4.704 

7.056 

9.408 

11.760 

14.112 

16.464 

18.817 

21.169 

18° 

72° 

2.295 

4.591 

6.886 

9.181 

11.476 

13.772 

16.067 

18.362 

20.657 

19° 

71° 

2.236 

4.471 

6.707 

8.943 

11.178 

13.414 

15.650 

17.885 

20.121 

20° 

70° 

2.173 

4.347 

6.520 

8.693 

10.867 

13.040 

15.213 

17.387 

19.560 

21° 

69° 

2.108 

4.217 

6.325 

8.434 

10.542 

12.650 

14.759 

16.867 

18.975 

22° 

68° 

2.041 

4.082 

6.123 

8.163 

10.204 

12.245 

14.286 

16.327 

18.368 

23° 

67° 

1.971 

3.942 

5.912 

7.883 

9.854 

11.825 

13.796 

15.767 

17.737 

24° 

66° 

1.898 

3.797 

5.695 

7.594 

9.492 

11.390 

13.289 

15.187 

17.086 

25° 

65° 

1.824 

3.647 

5.471 

7.295 

9.118 

10.942 

12.766 

14.589 

16.413 

26° 

64° 

1.747 

3.493 

5.240 

6.987 

8.734 

10.480 

12.227 

13.974 

15.720 

27° 

63° 

1.668 

3.335 

5.003 

6.670 

8.338 

10.006 

11.673 

13.341 

15.008 

28° 

62° 

1.587 

3.173 

4.756 

6.346 

7.932 

9.519 

11.105 

12.692 

14.278 

29° 

61° 

1.503 

3.007 

4.510 

6.014 

7.517 

9.021 

10.524 

12.028 

13.531 

30° 

60° 

1.419 

2.837 

4.256 

5.674 

7.093 

8.511 

9.930 

11.348 

12.767 

31° 

59° 

1.332 

2.664 

3.996 

5.328 

6.660 

7.992 

9.324 

10.656 

11.987 

32° 

58° 

1.244 

2.487 

3.731 

4.975 

6.218 

7.462 

8.706 

9.950 

11.193 

33° 

57° 

1.154 

2.308 

3.462 

4.616 

5.770 

6.924 

8.078 

9.232 

10.386 

34° 

56° 

1.063 

2.126 

3.188 

4.251 

5.314 

6.377 

7.440 

8.502 

9.565 

35° 

55° 

0.970 

1.941 

2.911 

3.881 

4.852 

5.822 

6.792 

7.763 

8.733 

36° 

54° 

0.877 

1.753 

2.630 

3.507 

4.384 

5.260 

6.137 

7.014 

7.890 

37° 

53° 

0.782 

1.564 

2.346 

3.128 

3.910 

4.692 

5.474 

6.256 

7.038 

38° 

52° 

0.686 

1.373 

2.059 

2.745 

3.432 

4.118 

4.805 

5.491 

6.177 

39° 

51° 

0.590 

1.180 

1.767 

2.360 

2.949 

3.539 

4.129 

4.719 

5.309 

40° 

50° 

0.493 

0.985 

1.478 

1.971 

2.463 

2.956 

3.449 

3.941 

4.434 

41° 

49° 

0.395 

0.790 

1.184 

1.579 

1.974 

2.369 

2.764 

3.159 

3.554 

42° 

48° 

0.297 

0.593 

0.890 

1.186 

1.483 

1.779 

2.076 

2.372 

2.669 

43° 

47° 

0.198 

0.396 

0.594 

0.792 

0.990 

1.187 

1.385 

1.583 

1.781 

44° 

46° 

0.099 

0.198 

0.297 

0.396 

0.495 

0.594 

0.693 

0.792 

0.891 

45° 

45° 

0.000 

0.000 

0.000 

0.000 

0.000 

0.000 

0.000 

0.000 

0.000 

TABLE  V.  Correction  for  Altitude,  on  account  of  Decrease  of  Gravity  from  level  of  sea  upward 
into  the  Atmoa.     To  be  applied  to  Height  obtained  from  Barom.  Observ.,  see  par,  95,  p.  68. 


Obtained 
Height 
in  Feet. 

Correc.  to 
be  added. 
Feet. 

Obtained 
Height 
in  Feet. 

Correc.  to 
be  added. 
Feet. 

Obtsiind 
Height 
in  Feet. 

Correc.  to 
be  added. 
Feet. 

Obtain'd 
Height 
in  Feet. 

Correc.  to 
be  added. 
Feet. 

Obtain'd 
Height 
in  Feet. 

Dorrec.  to 
be  added. 
Feet. 

200 

0.502 

5200 

14.303 

10200 

30.492 

15200 

49.087 

20200 

69.876 

400 

1.008 

5400 

14.905 

10400 

31.196 

15400 

49.880 

20400 

70.959 

600 

1.518 

5600 

15.511 

10600 

31.897 

15600 

50.677 

20600 

71.851 

800 

2.032 

5800 

16.120 

10800 

32.602 

15800 

51.478 

20800 

72.748 

1000 

2.550 

6000 

16.734 

11000 

33.312 

16000 

52.282 

21000 

73.649 

1200 

3.071 

6200 

17.351 

11200 

34.024 

16200 

53.092 

21200 

74.553 

1400 

3.596 

6400 

17.972 

11400 

34.741 

16400 

53.904 

21400 

75.461 

1600 

4.125 

6600 

18.597 

11600 

35.462 

16600 

54.721 

21600 

76.374 

1800 

4.658 

6800 

19.225 

11800 

36.186 

16800 

55.541 

21800 

77.289 

2000 

5.195 

7000 

19.858 

12000 

36.914 

17000 

56.365 

22000 

78.209 

2200 

5.735 

7200 

20.494 

12200 

37.646 

17200 

57.193 

22200 

79.133 

2400 

6.280 

7400 

21.134 

12400 

38.382 

17400 

58.024 

22400 

80.060 

2600 

6.828 

7600 

21.778 

12600 

39.122 

17600 

58.860 

22600 

80.991 

2800 

7.380 

7800 

22.426 

12800 

39.866 

17800 

59.699 

22800 

81.926 

3000 

7.936 

8000 

23.733 

13000 

40.613 

18000 

60.542 

23000 

82.865 

3200 

8.496 

8200 

24.165 

13200 

41.364 

18200 

61.389 

23200 

83.808 

3400 

9.059 

8400 

24.392 

13400 

42.119 

18400 

62.240 

23400 

84.755 

3600 

9.627 

8600 

25.055 

13600 

42.878 

18600 

63.095 

23600 

85.705 

3800 

10.198 

8800 

25.722 

13800 

43.641 

18800 

63.953 

23800 

86.659 

4000 

10.773 

9000 

26.393 

14000 

44.407 

19000 

64.815 

24000 

87.617 

4200 

11.352 

9200 

27.068 

14200 

45.177 

19200 

65.681 

24200 

88.579 

4400 

11.934 

9400 

27.746 

14400 

45.952 

19400 

66.565 

24400 

89.545 

4600 

12.521 

9600 

28.428 

14600 

46.730 

19600 

67.425 

24600 

90.514 

4800 

13.111 

9800 

29.115 

14800 

47.512 

19800 

68.303 

24800 

91.488 

5000 

13.705 

10000 

29.804 

15000 

48.297 

20000 

69.184 

25000 

92.465 

TABLE  VI.    For  the  conversion  of  French  into  English)  and  English,  into  French  measures. 


French 
Millime- 
tres. 

English 
Inches. 

English 
Inches. 

French 
Millimetres. 

French 
Metres. 

English 

Feet. 

English 

Feet. 

French 

Metres. 

1 

0.03937079 

1 

25.39954 

1 

3.2808992 

1 

0.30479449 

2 

0.07874158 

2 

50.79908 

2 

6.5617984 

2 

0.60958898 

3 

0.11811237 

3 

7&.  19862 

3 

9.8426976 

3 

0.91438347 

4 

0.15748316 

4 

101.59816 

4 

13.1235968 

4 

1.21917796 

5 

0.19685395 

5 

126.99770 

5 

16.4044960 

5 

1.52397245 

6 

0.23622474 

6 

152.39724 

6 

19.6853952 

6 

1.82876694 

7 

0.27559553 

7 

177.79678 

7 

22.9662944 

7 

2.13356143 

8 

0.31496632 

8 

203.19632 

8 

26.2471936 

8 

2.43835592 

9 

0.35433711 

9 

228.59586 

9 

29.5280928 

9 

2.74315041 

720 

28.34697 

27 

685.78758 

1  Paris  or  old  French  Foot  =  1.065765  English  Foot. 

730 

28.74068 

28 

711.18712 

1    "            "        "       Inch  rr  1.065765      "        Inch. 

740 
750 

760 

29.13438 
29.52809 
29.92180 

29 
30 
31 

736.58666 
761.98620 
787.38574 

1    "           "        "       Line  =  0.088814     "           « 
1  French  Litre  —  61.0275  English  cubic  Inches. 
1  Engl.  Wine  Gallon  =  231.044  Engl.  cubic  Inches. 
1  Entrl.  cubic  Inch  —  0.00432818  En^l   Win»  fi«l 

1  French  Gramme  Weight  r:  ljl433  Engl.  grains.      1  Eng.  cub.  In.:=252.458  Eng.  grains  of  Water  of  62°. 
1  Fr.  Kilogramme  =  2.2047  Engl.  pounds  Av.d.p.      1000  Eng.  gr's  Water  of  62°  =  3.961054  Eng.  cub.  In. 

TABLE   VII.     Giving  the   Maximum  Tension  or   Elastic  Force   of   Vapor  of   Water 
for  every  0.2  degree  from  214°  to  185°.     Pn.  par.  87  page  58,  and  par.  139  page  94. 


Temp. 
Fah. 

Max.  Tens, 
nch.  Merc. 

Differ- 
ences. 

Temp. 
Fah. 

Max.  Tens, 
nch.  Merc. 

Differ- 
ences. 

Temp. 
Fan. 

Max.  Tens. 
nch.  Merc. 

Differ- 
ences. 

214°.0 

213°.8 
213°.6 
213°.4 
213°.2 
213°.0 

31.132 
31.009 
30.887 
30.765 
30.643 
30.522 

0.123 
0.122 
0.122 
0.122 
0.121 
0.121 

204°.0 

203°.8 
203°.6 
203°.4 
203°.2 
203°.0 

25.468 
25.364 
25.261 
25.158 
25.055 
24.952 

0.104 
0.103 
0.103 
0.103 
0.103 
0.102 

194°.0 

193°.8 
193°.6 
193°.4 
193°.2 
193°.0 

20.687 
20.600 
20.513 
20.426 
20.340 
20.254 

0.087 

0.087 
0.087 
0.086 
0.086 
0.086 

212°.8 
212°.6 
212°.4' 
212°.2 
212°.0 

30.401 
30.281 
30.161 
30.041 

29.922 

0.120 
0.120 
0.120 
0.119 
0.119 

202°.  8 
202°.  6 
202°.  4 
202°.  2 
202°.0 

24.850 
24.748 
24.646 
24.545 
24.444 

0.102 
0.102 
0.101 
0.101 
0.101 

192°.8 
192°.6 
192°.4 
192°.2 
192°.0 

20.168 
20.082 
19.997 
19.912 
19.827 

0.086 
0.085 
0.085 
0.085 
0.084 

11 

29.803 
29.685 
29.567 
29.449 
29.332 

0.118 
0.118 
0.118 
0.117 
0.117 

20P.8 
20P.6 
20P.4 
20P.2 
20P.O 

24.343 
24.243 
24.144 
24.045 
23.946 

0.100 
0.099 
0.099 
0.099 
0.099 

i9io!e 

19P.4 
19P.2 

19.743 
19.659 
19.575 
19.492 
19.409 

0.084 
0.084 
0.083 
0.083 
0.083 

210°.8 
210°.  6 
210°.4 
210°.2 
210°.0 

29.215 
29.099 
28.983 
28.868 
28.753 

0.116 
0.116 
0.115 
0.115 
0.115 

200°.8 
200°.6 
200°.4 
200°.2 
200°.0 

23.847 
23.749 
23.651 
23.553 
23.456 

0.098 
0.098 
0.098 
0.097 
0.097 

190°.  8 
190°.6 
190°.4 
190°.2 
190°.0 

19.326 
19.243 
19.161 
19.079 
18.997 

0.083 
0.082 
0.082 
0.082 
0.081 

209°.  8 
209°.6 
209°.4 
209°.2 
209°.0 

28.638 
28.524 
28.410 
28.296 
28.183 

0.114 
0.114 
0.114 
0.113 
0.113 

199°.8 
199°.6 
199°.4 
199°.2 
199°.0 

23.359 
23.262 
23.166 
23.070 
22.974 

0.097 
0.096 
0.096 
0.096 
0.095 

189°.8 
189°.6 
189°.4 
189°.2 
189°.0 

18.916 
18.835 
18.754 
18.673 
18.593 

0.081 
0.081 
0.081 
0.080 
0.080 

208°.  8 
208°.  6 
208°.4 
208°.2 
208°.0 

28.070 
27.958 
27.846 
27.734 
27.622 

0.112 
0.112 
0.112 
0.112 
0.111 

198°.8 
198°.6 
198°.4 
198°.  2 
198°.0 

22.879 
22.784 
22.689 
22.595 
22.501 

0.095 
0.095 
0.094 
0.094 
0.094 

188°.  8 
188°.6 
188°.4 
188°.2 
188°.0 

18.513 
18.434 
18.355 
18.276 
18.197 

0.079 
0.079 
0.079 
0.079 
0.079 

207°.  8 
207°.6 
207°.4 
207°.2 
207°.0 

27.511 
27.400 
27.290 
27.180 
27.070 

'0.111 
0.110 
0.110 
0.110 
0.109 

197°.8 
197°.6 
197°.4 
197°.2 
197°.0 

22.407 
22.313 
22.220 
22.127 
22.035 

0.094 
0.093 
0.093 
0.092 
0.092 

187°.8 
187°.6 
187°.4 
187°.2 
187°.0 

18.118 
18.040 
17.962 
17.884 
17.807 

0.078 
0.078 
0.078 
0.077 
0.077 

206°.  8 
206°.  6 
206°.4 
206°.2 
206°.0 

26.961 
26.852 
26.743 
26.635 
26.527 

0.109 
0.109 
0.108 
0.108 
0.107 

196°.8 
196°.  6 
196°.4 
196°.2 
196°.0 

21.943 
21.851 
21.760 
21.669 
21.578 

0.092 
0.091 
0.091 
0.091 
0.090 

186°.8 
186°.6 
186°.4 
186°.2 
186°,0 

o 

17.730 
17.654 
17.578 
17.502 
17.426 

0.076 
0.076 
0.076 
0.076 
0.076 

205°.8 
205°.6 
205°.  4 
205°.2 
205°.0 

26.420 
26.313 
26.206 
26.100 
25.994 

0.107 
0.107 
0.106 
0.106 
0.106 

195°.8 
195°.6 
195°.4 
195°.2 
195°.0 

21.488 
21.398 
21.308 
21.218 
21.128 

0.090 
0.090 
0.090 
0.090 
0.089 

185    8 
185°.6 
185°.4 
185°.2 
185°.0 

17.350 
17.274 
17.199 
17.124 
17.049 

0.076 
0.075 
0.075 
0.075 

204°.  8 
204°.  6 

25.888 
25.782 

0.106 

ft   -IAK 

194°.8 
194°.6 

21.039 
20.950 

0.089 
0  088 

204°.4 

25.677 

v.  1UO 

194°.4 

20.862 

U.UOo 

204°.  2 

25.572 

0.105 
0  104. 

194°.2 

20.774 

0.088 
0  087 

204°.0 

25.468 

U.  J.UTC 

194°.0 

20.687 

V.  vO  t 

TABLE  VIII.     Giving  the,  Maximum  Tension  or  Elastic  Force  of  Vapor  of  Water, 
for  every  degree  from  185°  to  104°.     Pn.  par.  87  page  58,  and  par.  139  page  94. 


Temp. 
Fah. 

Max.  Tens. 
Inch.  Merc. 

Differ- 
ences. 

Temp. 
Fab. 

Max.  Tens. 
Inch.  Merc. 

Differ- 
ences. 

Temp. 
Fah. 

Max.  Tens. 
Inch.  Merc. 

Differ- 
ences. 

185° 

17.0492 

OCQQ 

158° 

9.1770 

»21Q9 

131° 

4.6252 

1221 

184° 
183° 
182° 
181° 
180° 
179° 
178° 
177° 
176° 
175° 
174° 
173° 
172° 
171° 
170° 
169° 
168° 
167° 
166° 
165° 
164° 
163° 
162° 
161° 
160° 
159° 

16.6804 
16.3182 
15.9626 
15.6135 
15.2709 
14.9346 
14.6045 
14.2805 
13.9625 
13.6504 
13.3442 
13.0438 
12.7491 
12.4601 
12.1767 
11.8988 
11.6263 
11.3591 
11.0971 
10.8402 
10.5883 
10.3413 
10.0991 
9.8617 
9.6289 
9.4007 

.3622 
.3556 
.3491 
.3426 
.3363 
.3301 
.3240 
.3180 
.3121 
.3062 
.3004 
.2947 
.2890 
.2834 
.2779 
.2725 
.2672 
.2620 
.2569 
.2519 
.2470 
.2422 
.2374 
.2328 
.2282 
.2237 

157° 
156° 
155° 
154° 
153° 
152° 
151° 
150° 
149° 
148° 
147° 
146° 
145° 
144° 
143° 
142° 
141° 
140° 
139° 
138° 
137° 
136° 
135° 
134° 
133° 
132° 

8.9578 
8.7431 
8.5328 
8.3269 
8.1253 
7.9281 
7.7349 
7.5456 
7.3602 
7.1787 
7.0010 
6.8271 
6.6568 
6.4901 
6.3269 
6.1672 
6.0109 
5.8580 
5.7084 
5.5621 
5.4190 
5.2791 
5.1423 
5.0086 
4.8779 
4.7501 

.2147 
.2103 
.2059 
.2016 
.1972 
.1932 
.1893 
.1854 
.1815 
.1777 
.1739 
.1703 
.1667 
.1632 
.1597 
.1563 
.1529 
.1496 
.1463 
.1431 
.1399 
.1368 
.1337 
.1307 
.1278 
.1249 

130° 
129° 
128° 
127° 
126° 
125° 
124° 
123° 
122° 
121° 
120° 
119° 
118° 
117° 
116° 
115° 
114° 
113° 
112° 
111° 
110° 
109° 
108° 
107° 
106° 
105° 

4.5031 
4.3838 
4.2673 
4.1534 
4.0421 
3.9334 
3.8273 
3.7237 
3.6214 
3.5224 
3.4257 
3.3313 
3.2392 
3.1493 
3.0615 
2.9758 
2.8922 
2.8107 
2.7313 
2.6538 
2.5782 
2.5044 
2.4324 
2.3622 
2.2937 
2.2269 

.1193 
.1165 
.1139 
.1113 
.1087 
.1061 
.1036 
.1013 
.0990 
.0967 
.0944 
.0921 
.0899 
.0878 
.0857 
.0836 
.0815 
.0794 
.0775 
.0756 
.0738 
.0720 
.0702 
.0685 
.0668 
.0652 

TABLE  IX.     Giving  the  Maximum  Tension  or  Elastic  Force  of  Vapor  of  Water, 
for  every  0.2  degree  from  104°  to  0°,  and  for  every  degree  from  0°  to —  31°. 


Temp. 
Fah. 

Max.  Tens. 
Inch.  Merc. 

Differ- 
ences. 

Temp. 
Fah. 

Max.  Tens. 
Inch.  Merc. 

Differ- 
ences. 

Temp. 
Fah. 

Max.  Tens. 
Inch.  Merc. 

Differ- 
ences. 

104°.0 

103°.8 
103°.  6 
103°.4 
103°.2 
103°.0 
102°.8 
102°.6 
102°.4 
102°.2 
102°.0 
101°.8 
101°.6 
101°.4 
101°.2 
101°.0 
100°.8 
100°.6 
100°.4 
100°.2 
100°.0 

2.1617 
2.1489 
2.1362 
2.1235 
2.1109 
2.0983 
2.0858 
2.0734 
2.0611 
2.0488 
2.0366 
2.0244 
2.0123 
2.0003 
1.9883 
1.9764 
1.9646 
1.9528 
1.9411 
1.9294 
1.9178 

.0128 
.0127 
.0127 
.0126 
.0126 
.0125 
.0124 
.0123 
.0123 
.0122 
.0122 
.0121 
.0120 
.0120 
.0119 
.0118 
.0118 
.0117 
.0117 
.0116 

100°.0 

99°.8 
99°.6 
99°.4 
9-9°.  2 
99°.0 
98°.8 
98°.6 
98°.4 
98°.2 
98°.0 
97°.8 
97°.6 
97°.4 
97°.2 
97°.0 
96°.  8 
96°.  6 
96°.  4 
96°.2 
96°.0 

1.9178 
1.9063 
1.8948 
1.8833 
1.8719 
1.8606 
1.8494 
1.8382 
1.8271 
1.8161 
1.8051 
1.7942 
1.7833 
1.7724 
1.7616 
1.7509 
1.7402 
1.7296 
1.7190 
1.7085 
1.6981 

.0115 
.0115, 
.0115 
.0114 
.0113 
.0112 
.0112 
.0111 
.0110 
.0110 
.0109 
.0109 
.0109 
.0108 
.0107 
.0107 
.0106 
.0106 
.0105 
.0104 

96°.0 

95°.  8 
95°.6 
95°.4 
95°.2 
95°.0 
94°.  8 
94°.  6 
94°.  4 
94°.  2 
94°.0 
93°.8 
93°.6 
93°.4 
93°.2 
93°.0 
92°.8 
92°.  6 
92°.4 
92°.2 
92°.0 

1.6981 
1.6878 
1.6775 
1.6672 
1.6570 
1.6468 
1.6366 
1.6265 
1.6165 
1.6066 
1.5967 
1.5869 
1.5771 
1.5674 
1.5577 
1.5480 
1.5384 
1.5289 
1.5194 
1.5100 
1.5006 

.0103 
.0103 
.0103  I 
.0102 
.0102 
.0102 
.0101 
.0100 
.0099 
.0099 
.0098 
.0098 
.0097 
.0097 
.0097 
.0096 
.0095 
.0095 
.0094 
.0094 

TABLE  IX  (Continued).  Giving  the  Maximum  Tension  or  Elastic  Force  of  Vapor  of  Water. 


Temp. 
Fah. 

Max.  Tens. 
Inch.  Merc. 

UifllT- 
ences. 

Temp. 
Fah 

Max.  Tens, 
nch.  Merc. 

Diiler- 
ences. 

Temp. 
Fah 

Max.  Tens, 
nch.  Merc. 

DilTer- 
ences. 

92°.0 

91°.8 
91°.6 
91°.4 
91°.2 
91°.0 
90°.8 
90°.  6 
90°.4 
90°.2 
90°.0 
89°.8 
89°.6 
89°.4 
89°.2 
89°.0 
88°.  8 
88°.  6 
88°.  4 
88°.2 
88°.0 
87°.8 
87°.6 
87°.4 
87°.2 
87°.0 
86°.  8 
86°.  6 
86°.4 
86°.2 
86°.0 
85°.8 
85°.6 
85°.4 
85°.2 
85°.0 
84°.  8 
84°.6 
84°.  4 
84°.  2 
84°.0 
83°.8 
83°.6 
83°.  4 
83°.2 
83°.0 
82°.  8 
82°.  6 
82°.4 
82°.  2 
82°.0 
81°.8 
81°.6 
81°.4 
81°.2 
81°.0 

1.5006 

1.4913 
1.4821 
1.4729 
1.4637 
1.4545 
1.4454 
1.4364 
1.4274 
1.4185 
1.4096 
1.4008 
1.3921 
1.3834 
1.3747 
1.3661 
1.3575 
1.3489 
1.3404 
1.3319 
1.3235 
1.3152 
1.3069 
1.2986 
1.2904 
1.2822 
1.2741 
1.2660 
1.2580 
1.2500 
1.2421 
1.2342 
1.2263 
1.2185 
1.2107 
1.2030 
1.1953 
1.1877 
1.1801 
1.1726 
1.1651 
1.1576 
1.1502 
1.1428 
1.1354 
1.1281 
1.1208 
1.1136 
1.1064 
1.0993 
1.0922 
1.0851 
1.0781 
1.0711 
1.0641 
1.0572 

.0093 

.0092 
.0092 
.0092 
.0092 
.0091 
.0090 
.0090 
.0089 
.0089 
.0088 
.0087 
.0087 
.0087 
.0086 
.0086 
.0086 
.0085 
.0085 
.0084 
.0083 
.0083 
.0083 
.0082 
.0082 
.0081 
.0081 
.0080 
.0080 
.0079 
.0079 
.0079 
.0078 
.0078 
.0077 
.0077 
.0076 
.0076 
.0075 
.0075 
.0075 
.0074 
.0074 
.0074 
.0073 
.0073 
.0072 
.0072 
.0071 
.0071 
.0071 
.0070 
.0070 
.0070 
.0069 

81°.0 

80°.8 
80°.  6 
80°.4 
80°.2 
80°.0 
79°.8 
79°.6 
79°.4 
79°.2 
79°.0 
78°.  8 
78°.6 
78°.4 
78°.2 
78°.0 
77°.8 
77°.6 
77°.4 
77°.2 
77°.0 
76°.8 
76°.6 
76°.4 
76°.2 
76°.0 
75°.8 
75°.6 
75°.4 
75°.2 
75°.0 
74°.8 
74°.6 
74°.  4 
74°.2 
74°.0 
73°.8 
73°.6 
,73°.4 
73°.2 
73°.0 
72°.8 
72°.6 
72°.4 
72°.2 
72°.0 
71°.8 
71°.6 
71°.4 
71°.2 
71°.0 
70°.8 
70°.6 
70°,  4 
70°.2 
70°.0 

1.0572 
1.0503 
1.0435 
1.03G7 
1.0300 
1.0233 
1.0166 
1.0100 
1.0034 
0.9968 
0.9903 
0.9838 
0.9774 
0.9710 
0.9646 
0.9583 
0.9520 
0.9457 
0.9395 
0.9333 
0.9272 
0.9211 
0.9150 
0.9089 
0.9028 
0.8968 
0.8909 
0.8850 
0.8792 
0.8734 
0.8676 
0.8618 
0.8560 
0.8503 
0.8446 
0.8390 
0.8334 
0.8279 
0.8224 
0.8169 
0.8114 
0.8060 
0.8006 
0.7952 
0.7898 
0.7845 
0.7792 
0.7740 
0.7688 
0.7636 
0.7585 
0.7534 
0.7483 
0.7432 
0.7381 
0.7331 

.0069 
.0068 
.0068 
.0067 
.0067 
.0067 
.0066 
.0066 
.0066 
.0065 
.0065 
.0064 
.0064 
.0064 
.0063 
.0063 
.0063 
.0062 
.0062 
.0061 
.0061 
.0061 
.0061 
.0061 
.0060 
.0059 
.0059 
.0058 
.0058 
.0058 
.0058 
.0058 
.0057 
.0057 
.0056 
.0056 
.0055 
.0055 
.0055 
.0055 
.0054 
.0054 
.0054 
.0054 
.0053 
.0053 
.0052 
.0052 
.0052 
.0051 
.0051 
.0051 
.0051 
.0051 
.0050 

70°.0 

69°.8 
69°.6 
69°.4 
69°.2 
69°.0 
68°.8 
68°.6 
68°.4 
68°.2 
68°.0 
67°.8 
67°.6 
67°.4 
67°.2 
67°.0 
66°.8 
66°.6 
66°.4 
66°.2 
66°.0 
65°.8 
65°.6 
65°.4 
65°.2 
65°.0 
64°.  8 
64°.6 
64°.  4 
64°.2 
64°.0 
63°.8 
63°.6 
63°.4 
63°.2 
63°.0 
62°.8 
62°.6 
62°.4 
62°.2 
62.°0 
61°.8 
61°.6 
61°.4 
6P.2 
61°.0 
60°.8 
60°.  6 
60°.4 
60°.  2 
60°.0 
59°.8 
59°.6 
59°.4 
59°.2 
59°.0 

0.7331 

0.7281 
0.7232 
0.7183 
0.7134 
0.7085 
0.7036 
0.6988 
0.6941 
0.6894 
0.6847 
0.6800 
0.6754 
0.6708 
0.6662 
0.6616 
0.6570 
0.6525 
0.6480 
0.6435 
0.6391 
0.6347 
0.6303 
0.6260 
0.6217 
0.6174 
0.6131 
0.6088 
.0.6046 
0.6004 
0.5962 
0.5921 
0.5880 
0.5839 
0.5798 
0.5758 
0.5718 
0.5678 
0.5638 
0.5599 
0.5560 
0.5521 
0.5482 
0.5443 
0.5405 
0.5367 
0.5329 
0.5291 
0.5254 
0.5217 
0.5180 
0.5143 
0.5107 
0.5071 
0.5035 
0.4999 

.0050 

.0049 
.0049 
.0049 
.0049 
.0049 
.0048 
.0047 
.0047 
.0047 
.0047 
.0046 
.0046 
.0046 
.0046 
.0046 
.0045 
.0045 
.0045 
.0044 
.0044 
•0044 
.0043 
.0043 
.0043 
.0043 
.0043 
.0042 
.0042 
.0042 
.0041 
.0041 
.0041 
.0041 
.0040 
.0040 
.0040 
.0040 
.0039 
.0039 
.0039 
.0039 
.0039 
.0038 
.0038 
.0038 
.0038 
.0037 
.0037 
.0037 
.0037 
.0036 
.0036 
.0036 
.0036 

TABLE  IX.  (Continued).  Giving  the  Maximum  Tension  or  Elastic  Force  of  Vapor  of  Water. 


Temp. 
Fah. 

Max.  Tens. 
Inch.  Merc. 

Differ- 
ences. 

Temp. 
Fan. 

Max.  Tens- 
Inch.  Merc- 

Differ- 
ences. 

Temp. 
Fah. 

Max.  Tens. 
Inch.  Merc. 

Differ- 
ences. 

59°.0 

58°.8 
58°.6 
58°.4 
58°.2 
58°.0 
57°.8 
57°.6 
57°.4 
57°.2 
57°.0 
5G°.8 
56°.6 
56°.4 
56°.2 
56°.0 
55°.8 
55°.6 
55°.4 
55°.2 
55°.0 
54°.  8 
54°.6 
54°.  4 
54°.2 
54°.0 
53°.8 
53°.  6 
53°.4 
53°.2 
53°.0 
52°.8 
52°.6 
52°.4 
52°.2 
52°.0 
51°.8 
51°.6 
51°,4 
51°.2 
51°.0 
50°.  8 
50°.6 
50°.  4 
50°.  2 
50°.0 
49°.8 
49°.  6 
49°.4 
49°.2 
49°.0 
48°.  8 
48°.6 
48°.4 
48°.  2 
48°.0 

0.4999 

0.4964 
0.4929 
0.4894 
0.4859 
0.4824 
0.4790 
0.4756 
0.4722 
0.4688 
0.4655 
0.4622 
0.4589 
0.4556 
0.4523 
0.4491 
0.4459 
0.4427 
0.4395 
0.4363 
0.4331 
0.4299 
0.4268 
0.4237 
0.4207 
0.4177 
0.4147 
0.4117 
0.4087 
0.4057 
0.4028 
0.3999 
0.3970 
0.3941 
0.3912 
0.3883 
0.3855 
0.3827 
0.3799 
0.3771 
0.3743 
0.3716 
0.3689 
0.3662 
0.3635 
0.3608 
0.3581 
0.3555 
0.3529 
0.3503 
0.3477 
0.3451 
0.3426 
0.3401 
0.3376 
0.3351 

.0035 
.0035 
.0035 
.0035 
.0035 
.0034 
.0034 
.0034 
.0034 
.0033 
.0033 
.0033 
.0033 
.0033 
.0032 
.0032 
.0032 
.0032 
.0032 
.0032 
.0032 
.0031 
.0031 
.0030 
.0030 
.0030 
.0030 
.0030 
.0030 
.0029 
.0029 
.0029 
.0029 
.0029 
.0029 
.0028 
.0028 
.0028 
.0028 
.0028 
.0027 
.0027 
.0027 
.0027 
.0027 
.0027 
.0026 
.0026 
.0026 
.0026 
.0026 
.0025 
.0025 
.0025 
.0025 

48°.0 

47°.8 
47°.6 
47°.4 
47°.2 
47°.0 
46°.  8 
46°.  6 
46°.4 
46°.  2 
46°.0 
45°.8 
45°.6 
45°.4 
45°.2 
45°.0 
44°.  8 
44°.  6 
44°.4 
44°.2 
44°.0 
43°.8 
43°.6 
43°.4 
43°.2 
43°.0 
42°.8 
42°.6 
42°.4 
42°,2 
42°.0 
41°.8 
41°.6 
41°.4 
41°.2 
41°.  0 
40°.8 
40°.  6 
40°.4 
40°.  2 
40°.0 
39°.8 
39°.6 
39°.4 
39°.2 
39°.0 
38°.8 
38°.6 
38°.4 
38°.2 
38°,0 
37°.8 
37°.  6 
37°.4 
37°.2 
37°.0 

0.3351 
0.3326 
0.3301 
0.3276 
0.3252 
0.3228 
0.3204 
0.3180 
0.3156 
0.3132 
0.3109 
0.3086 
0.3063 
0.3040 
0.3017 
0.2994 
0.2972 
0.2950 
0.2928 
0.2906 
0.2884 
0.2862 
0.2840 
0.2818 
0.2797 
0.2776 
0.2755 
0.2734 
0.2713 
0.2692 
0.2672 
0.2652 
0.2632 
0.2612 
0.2592 
0.2572 
0.2552 
0.2533 
0.2514 
0.2495 
0.2476 
0.2457 
0.2438 
0.2419 
0.2400 
0.2382 
0.2364 
0.2346 
0.2328 
0.2310 
0.2292 
0.2274 
0.2256 
0.2239 
0.2222 
0.2205 

.0025 
.0025 
.0025 
.0024 
.0024 
.0024 
.0024 
.0024 
.0024 
.0023 
.0023 
.0023 
.0023 
.0023 
.0023 
.0022 
.0022 
.0022 
.0022 
.0022 
.0022 
.0022 
.0022 
.0021 
.0021 
.0021 
.0021 
.0021 
.0021 
.0020 
.0020 
.0020 
.0020 
.0020 
.0020 
.0020 
.0019 
.0019 
.0019 
.0019 
.0019 
.0019 
.0019 
.0019 
.0018 
.0018 
.0018 
.0018 
.0018 
.0018 
.0018 
.0018 
.0017 
.0017 
.0017 

37.  °0 

36°.8 
36°.6 
36°.4 
36°.2 
36°.0 
35°.  8 
35°.6 
35°.4 
35°.2 
35°.0 
34°.8 
34°.  6 
34°.  4 
34°.2 
34°.0 
33°.8 
33°.6 
33°.4 
33°.  2 
33°.0 
32°.8 
32°.6 
32°.4 
32°.2 
32°.0 
31°.8 
31°.6 
31°.4 
31°.2 
31°.0 
30°.  8 
30°.6 
30°.4 
30°.  2 
30°.0 
29°.8 
29°.6 
29°.4 
29°.  2 
29°.0 
28°.  8 
28°.6 
28°.4 
28°.2 
28°.0 
27°.8 
27°.6 
27°.4 
27°.2 
27°.0 
26°.8 
26°.6 
26°.4 
26°.2 
26°.0 

0.2205 

0.2188 
0.2171 
0.2154 
0.2137 
0.2120 
0.2104 
0.2088 
0.2072 
0.2056 
0.2040 
0.2024 
0.2008 
0.1992 
0.1976 
0.1960 
0.1944 
0.1929 
0.1914 
0.1899 
0.1884 
0.1869 
0.1854 
0.1840 
0.1825 
0.1811 
0.1796 
0.1781 
0.1766 
0.1751 
0.1736 
0.1722 
0.1708 
0.1694 
0.1680 
0.1666 
0.1652 
0.1638 
0.1624 
0.1610 
0.1596 
0.1583 
0.1570 
0.1557 
0.1544 
0.1531 
0.1518 
0.1505 
0.1492 
0.1479 
0.1466 
0.1454 
0.1442 
0.1430 
0.1418 
0.1406 

.0017 
.0017 
.0017 
.0017 
.0017 
.0016 
.0016 
.0016 
.0016 
.0016 
.0016 
.0016 
.0016 
.0016 
.0016 
.0016 
.0015 
.0015 
.0015 
.0015 
.0015 
.0015 
.0015 
.0015 
.0015 
.0015 
.0015 
.0015 
.0015 
.0015 
.0014 
.0014 
.0014 
.0014 
.0014 
.0014 
.0014 
.0014 
.0014 
.0014 
.0013 
.0013 
.0013 
.0013 
.0013 
.0013 
.0013 
.0013 
.0013 
.0013 
.0012 
.0012 
.0012 
.0012 
.0012 

TABLE  IX.  (Continued).  Giving  the  Maximum  Tension  or  Elastic  Force  of  Vapor  of  Water. 


Temp. 
Fab. 

lax.  Tens, 
nch.  Merc. 

Differ- 
ences. 

Temp. 
Fan. 

Max.  Tens, 
nch.  Merc. 

Differ- 
ences. 

Temp. 
Fah. 

Max.  Tens, 
nch.  Merc. 

Differ- 
ences. 

26°.0 

25°.8 
25°.6 
25°.4 
25°.2 
25°.0 
24°.  8 
24°.  6 
24°.  4 
24°  2 
24°.0 
23°.8 
23°.6 
23°.4 
23°.2 
23°.0 
22°.8 
22°.6 
22°.4 
22°  2 
22°.0 

0.1406 

0.1394 
0.1382 
0.1370 
0.1358 
0.1346 
0.1334 
0.1322 
0.1310 
0.1299 
0.1288 
0.1277 
0.1266 
0.1255 
0.1244 
0.1233 
0.1222 
0.1211 
0.1200 
0.1189 
0.1178 

.0012 

.0012 
.0012 
.0012 
.0012 
.0012 
.0012 
.0012 
.0011 
.0011 
.0011 
.0011 
.0011 
.0011 
.0011 
.0011 
.0011 
.0011 
.0011 
.0011 
0010 

15°.0 

14°.8 
14°.6 
14°.4 
14°.2 
14°.0 
13°.8 
13°.6 
13°.4 
13°.2 
13°.0 
12°.8 
12°.6 
12°.4 
12°.2 
12°.0 
11°.8 
11°.6 
11°.4 
11°.2 
11°.0 

0.0858 
0.0850 
0.0842 
0.0834 
0.0826 
0.0818 
0.0810 
0.0803 
0.0796 
0.0789 
0.0782 
0.0775 
0.0768 
0.0761 
0.0754 
0.0747 
0.0740 
0.0733 
0.0726 
0.0719 
0.0713 

.0008 

.0008 
.0008 
.0008 
.0008 
.0008 
.0007 
.0007 
.0007 
.0007 
.0007 
.0007 
.0007 
.0007 
.0007 
.0007 
.0007 
•0007 
.0007 
.0006 
OOOfi 

4°.0 

3°.8 
3°.6 
3°.4 
3°.2 
3°.0 
2°.8 
2°.6 
2°  4 
2°!2 
2°.0 
1°.8 
1°.6 
1°.4 
1°.2 
1°.0 
0°.8 
0°.6 
0°.4 
0°.2 

o°.o 

0.0520 
0.0515 
0.0510 
0.0505 
0.0500 
0.0495 
0.0490 
0.0485 
0.0481 
0.0477 
0.0473 
0.0469 
0.0465 
0.0461 
0.0457 
0.0453 
0.0449 
0.0445 
0.0441 
0.0437 
0.0433 

.0005 

.0005 
.0005 
.0005 
.0005 
.0005 
.0005 
.0004 
.0004 
.0004 
.0004 
.0004 
.0004 
.0004 
.0004 
.0004 
.0004 
.0004 
.0004 
.0004 

21°.8 
21°.6 
21°.4 
21°.2 
21°.0 
20°.  8 
20°.6 

0.1168 
0.1158 
0.1148 
0.1138 
0.1128 
0.1118 
0.1108 

.0010 
.0010 
.0010 
.0010 
.0010 
.0010 

10°.8 
10°.6 
10°.4 
10°.2 
10°.0 
9°.8 
9°.6 

0.0707 
0.0701 
0.0695 
0.0689 
0.0683 
0.0677 
0.0671 

.0006 
.0006 
.0006 
.0006 
.0006 
.0006 

0° 

—1° 

2° 
3° 
4° 
5° 

0.0433 
0.0413 
0.0394 
0.0376 
0.0360 
0.0344 

.0020 
.0019 
.0018 
.0016 
.0016 
.0016 

20°.  4 
20°.2 
20°.0 
19°.8 
19°.6 
19°.4 
19°.2 
19°.0 
18°.8 
18°.6 
18°.4 
18°.2 
18°.0 
17°.8 
17°.6 
17°.4 

0.1098 
0.1088 
0.1078 
0.1068 
0.1058 
0.1048 
0.1039 
0.1030 
0.1021 
0.1012 
0.1003 
0.0994 
0.0985 
0.0976 
0.0967 
0.0958 

.uuiu 
.0010 
.0010 
.0010 
.0010 
.0010 
.0009 
.0009 
.0009 
.0009 
.0009 
.0009 
.0009 
.0009 
.0009 
.0009 

9°.4 

9°.2 
9°,0 

8°.8 
8°.6 
8°.4 
8°.2 
8°.0 
7°.8 
7°.6 
7°.4 
7°.2 
7°.0 
6°.8 
6°.  6 
6°.4 

0.0665 
0.0659 
0.0653 
0.0647 
0.0641 
0.0635 
0.0629 
0.0623 
0.0617 
0.0611 
0.0605 
0.0600 
0.0595 
0.0590 
0.0585 
0.0580 

.UUUo 
.0006 
.0006 
.0006 
.0006 
.0006 
.0006 
.0006 
.0006 
.0006 
.0006 
.0005 
.0005 
.0005 
.0005 
.0005 

—6° 
7° 
8° 
9° 
10° 

—11° 
12° 
13° 
14° 
15° 
—16° 
17° 
18° 
19° 
20° 

0.0328 
0.0313 
0.0299 
0.0285 
0.0272 

0.0259 
0.0247 
0.0236 
0.0225 
0.0215 

0.0205 
0.0196 
0.0187 
0.0178 
0.0170 

.0015 
.0014 
.0014 
.0013 
.0013 
.0012 
,0011 
.0011 
.0010 
.0010 
.0009 
.0009 
.0009 
.0008 
.0008 

17°.2 
17°.0 

16°.  8 
16°.6 
16°.4 
16°,2 
16°  .0 
15°.8 
15°.6 
15°.4 
15°.2 

0.0949 
0.0940 
0.0931 
0.0922 
0.0914 
0.0906 
0.0898 
0.0890 
0.0882 
0.0874 
0.0866 

.0009 
.0009 
.0009 
.0009 
.0008 
.0008 
.0008 
.0008 
.0008 
.0008 
.0008 
0008 

6°.2 
6°.0 

5°.8 
5°.  6 
5°.  4 
5°.  2 
5°.0 
4°.8 
4°.  6 
4°.4 
4°.  2 

0.0575 
0.0570 
0.0565 
0.0560 
0.0555 
0.0550 
0.0545 
0.0540 
0.0535 
0.0530 
0.0525 

.UOUo 
.0005 
.0005 
.0005 
.0005 
.0005 
.0005 
.0005 
.0005 
.0005 
.0005 
OOO1! 

—21° 
22° 
23° 
24° 
25° 
—26° 
27° 
28° 
29° 
30° 
31° 

0.0162 
0.0154 
0.0147 
0.0140 
0.0133 

0.0127 
0.0121 
0.0115 
0.0110 
0.0105 
0.0100 

.0008 
.0007 
.0007 
.0007 
.0006 

.0006 
.0006 
.0005 
.0005 
.0005 

15°.0 

0.0858 

4°.0 

0.0520 

JVC, 

/if? 

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